So I bought Concrete Donkey and Armageddon with my first points that I earned. ...Where do they show up for use? I can't use 'em in ranked matches, private matches nor on the campaign.

I r noob and I need halp!

You can set a match type for custom games in the help & options menu. You can add them and many other weapons and utilities there. Thats also where you can choose what Fort to use in multiplayer :)

ooooh. Huh. Okay then.

... I can set up custom games. Can I then open it to anyone on Live to join, like in a lobby then we get our play on?