Custom Games "Incompatible"?

it means you havent put the right objective things on your map for it to work in anything but Slayer.

You need to set the spawn points accordingly, most likely.

There should be an option when you press "X", and set the zombie spawners to "attackers" and the humans to "defenders."

Make sure you set the gametype to Infection before you start Forge. Then take whatever item it is you need from Forge's item menu (any item will work I believe, but for example, you'll probably want to use the Capture Plate for Assault gametypes, etc). Place it, press 'X' on it, go to 'Advanced', go to the list of labels, and add the label you need. I believe for Infection, the only label required is 'inf_haven'. I'm assuming this is supposed to be used on an object near where the humans spawn, so perhaps if you have a large single-object base for the humans, you could add the label to that. Experiment and find out, because I honestly haven't even tried making an Infection map yet. There's other Infection-related labels, but that's the only one that's required.

You might also have to change the indicated team for each of the respawn and initial spawn points, rather than have them neutral. Red is for humans and blue is for the Infected.

Well, I believe by going into a certain gametype for Forge, you unlock the necessary labels for that gametype. If the labels were there for Basic Editing and you added them, then you should be fine. But you can't just put down two capture stands and expect it to work, you have to indicate what that item is there for exactly through the labeling system. As well as the team color system, I think. I made a One-Bomb Assault map this way and it worked.