So here I am - again.
I've had some trouble with the Ez7z software. When ever I try to expand the mod-file (in this case the [TSL] Yet (not) Another Earth Maps Pack), I end up with six to seven files that all need additional unpacking, and this is also where Ez7z is of no further help. That's why I installed something called BetterZip. I then changed the extension on the mod-file from .civ5mod to .7z, and that seemed to work. The mod was expanded properly, and I put the files in a single folder before I dropped it into the MODS folder. Finally I launched the game, activated the mod, and everything looked just fine and dandy, until I realized that I couldn't find the "Custom Game" option (which I know I've seen before).
So my question is: Why is the "Custom Game" option not showing? It seems to me that I've done everything by the book, but I still can't access the mod despite having activated it in the Civ 5 menu.