Please (re)activate custom civilizations in multiplayer games (with crusade).
It is not possible to start a multiplayer game with an own custom civilization.
But that's what makes the game interesting.
You just can't choose it in the menu. There are only the standard civilizations.
Any upload (or download) to steam didn't help.
It seems that option is just de-activated.
(It won't be a problem if you have to upload your own race first)