As of B24, the Scenario Menu has replaced the Skirmish Menu and what used to be the Campaign screen. Only scenes that are compatible with the currently selected activity are shown. That's why a scene that you've just made doesn't show up in the menu on any activities.
The way the game determines if a scene is compatible with an activity is by scanning through the activity's Lua script and finding any calls to SceneMan:GetArea. It determines what area the script is looking for, then only shows scenes with those areas.
Areas can be placed using the Area editor without a hitch.

The default areas that ALL MAPS should have in order to be compatible with Vanilla content are as follows:

LZ Team 1-4 are where the respective teams are allowed to land on the map.
LZ All is a special zone that defines the zone that anything can land in in certain activities. For instance, in One Man Army, the AI can land anywhere in LZ All, as the player can move throughout the entire map. LZ All is useful if, for instance, you've made a map with a large pit in it - you don't want the AI landing in there, so just don't include it in LZ All or any Team LZs, and you're good to go.

In the future, there will likely be BZ Team1-4, defining where teams are allowed to build their bunkers. If you're into forward-compatibility, go ahead and put those into your maps - they'll just harmlessly sit there until they're in use, as the game doesn't do anything with extra Areas.

Hope this helps a bit. Let me know if you have any other questions regarding scenes and activities in B24.

This is what I thought I needed to do to some of those pesky invisible scenes. Thanks, TLB.

So after creating a scene in the editor I just need to add those areas? Do I need all of them, or just the ones for the scenario I want?

You should probably add all of them.

I made a scene in the scene editor and added all areas, but it still doesn't show up in the scenario battle menu. Why?

Check it has a position on the planet that isn't on top of some thing else. Try entering a random number to get it out in the open.

Aha! That was the problem. So from now on, you have to do some .ini edition before you can play?

Thanks!

Well, the scene you edited it from had one position on the planet, but there's no option to change it in the in-game editor, so it's just inherited.

I've created GAScripted activity without any areas defined on scene or obtained from script and CC offered me many available scenes to play it on instead of the single one for which it was designed and which is written in AddActivity. That means that Area matching is currently the ONLY method which CC uses to find a scene for an activity, even for scripted ones which specify their scenes explicitly, am I correct?

It's the only way I've got round it so far.

I guess there is some problem with zone scanning code, if I use --[[ ]]-- style comments at the beginning of my mission code it offers me all scenes available.