Whenever I try a Skirmish, it asks me to place things, but then when I'm done, nothing happens. It doesn't ask me to place an AI, and it doesn't place one itself, so I'm left on a blank map with nothing to do. Which seems especially odd, since it asks me what AI difficulty it wants me to have.

I've tried to find information about this, but Skirmish information is extremely scarce. I didn't find anything relevant on this forum, I didn't find anything at all on the Wiki about Skirmishes, the tutorial doesn't even mention other play types besides campaign, and there's nothing for this game even remotely close to a manual. I have other questions besides this, but it seems the only place I can get them answered is in the forum.

Wait, the enemies should begin sropping in from the sky in rockets and dropships

Check if your missions activities file is functional.

Could be it.

EDIT:
Something could have deleted out the spawns.

Nah, it probably is, otherwise it'd crash when you selected difficulty.
Just try waiting a while, listening carefully for the sound of dropship thrusters. Though admittedly alot of the invaders will be killed by their terrible piloting... If you really think nothing is happening then try death difficulty, there's a near constant stream of invaders.

if you dont place a brain there will be a message saying "PLACE A BRAIN FIRST MORON" or similar when you click "done building", that could be the problem if you arent paying attention.. otherwise, you might want to try re-installing, cause in every version of CC i've ever used the skirmish has worked fine (other than the "cant win" issue).

Hm..

I apologize if I am underestimating you, but did you press the "Done Building" button?

I like being absolutely sure.

that can be it,i always say"dont think too much and dont think less then normal,or something bad could happen"

yes

Skirmish: You click on it from the first menu that comes up when you start the game. Next, you select a map, team, and difficulty, and then start the skirmish. There's a black screen as the map loads. When it loads, it will either A, start in the BYOB (Build Your Own Base) phase, where you can place objects to make your custom survival fortress of awesome, or B, plunge you into a pre-made level/mission where you have to immediately begin playing. After you finish building your base or are plunged into the level, enemies will begin spawning. They do this in dropships and rockets that spawn above the map's visible Y range. If you have the sky blocked off, you probably won't see them, as their ships will either return immediately or drop off their troops and your enemies will be walking around in the atmosphere, trying to dig through whatever is obstructing their path to the brain. Enemies spawn at different rates for the different difficulties- eg, Easy is a spawninterval of 2000, 20 seconds between spawns, while Hard is 800, 8 seconds between spawns.

If, for some reason, enemies aren't spawning in skirmish and you have not obstructed their spawn points, then there's a problem. Either A, you're playing on some weird map where they all explode; B, you don't have any Activities enabled; C, you're just too inept to click a few times and start Skirmish.

Did you finish building - Check
Did you start - Check
Does Missions.rte/Activities.ini exist - ?
Does Missions.rte/Index.ini contain Includefile = Missions.rte/Activities.ini without a // in front?
Does Missions.rte/Activities.ini actually contain anything? (Base.rte activities don't do anything, it got patched a while ago, remember?)
Did you build an enormous wall over the top of the map, blocking spawns.
do you have a timescale > 0.01X (Ctrl + P on windows ??? on mac)
is CC the actively selected window (if in windowed mode)
Have you played CC on one of the default vanilla maps in skirmish?
Are you using a mod that includes a new activities.ini, that includes no spawn, or a spawn that always dies (watch the kill counts on the top right)

If none of these are issues, then you must un-install, and re-install your game. If this does not correct the issue, contact someone on the development team directly in the form of a PM. If they cannot fix it, then I don't know what to say, quit trying to play on Linux/amiga/Unix/tandy/C64/Hal9000/Wopr/Xbox/NES/table/emachine/robot, as they are all either fictional or nonfunctional.

Maybe I didn't wait long enough. I think I selected Easy, but then while standing around waiting what to do, I might not have waited a full 20 seconds. After all, 20 seconds of nothing happening is enough to make anyone think that something's fishy. So, I'll look into that and get back to you.

Alright, I tried it. You were right. I just wasn't waiting long enough for the dropships to arrive, and set the difficulty too low. I feel a little silly now, but then, like I said, it's not like there's a manual for these things (yet?)

yeah, a proper manual will come eventually, hopefully spattered with prom's art. In any case, the spawninterval is 20000 not 2000, the time is in MS for if you want to tweak your activities. glad the issue wasnt real though. see if you can get a hold of prom grounds for a hard scene, grasslands and horizlands are too easy even on death because of how wide they are.
(im on borrowed internet for anyone wondering why the hell i can post)

What the ♥♥♥♥ is this ♥♥♥♥ ^?

Geti had a teeny tiny message.

Haven't you ever heard of Loud Howard?