Go to mods, enable the mod(s) you want, click on next, then click on single player, not on back. This way you can play a game with the mods you selected. Hope this helps...

same problem for me... going thru mods, enabled it, then click single player without going back. weird thing is this used to work fine, I've played a few games as Gaben already but today went to play another and he doesn't show up in the list. other modded civ leaders I try work fine but not gaben anymore. I tried removing mod database and re-subscribing. it re-downloads it and everything but still doesnt show up in the list for some reason. I'm on Mac OS X for what its worth...