Hey all, I own GOTY edition of Fallout 3 on PC.  I hit level 20 before the Tranquility Lane mission.  I'm now heading to Vault 87 to look for the G.E.C.K.  It was my understanding that the level Cap was raised from 20-30 with the addition of the DLC and that I'd be able to continue leveling even if I haven't finished the main quest but I haven't earned experience points for anything since I reached level 20.  Whether it be killing an enemy, selling scrap metal, or completing a mission, experience points are never gained.  Has anyone else had this problem?  Any suggestions?  Please try not to spoil any future story, thanks.

 By the sounds of it the DLC hasn't activated.  Do you have various radio signals for the DLC content?  If not, try going to the game launcher and there should be a list of addons buried in the menu, simply need to check the boxes next to all the DLC content so the game loads it up properly.

I could be wrong. it's been a long ass time since i played. but i think you need to finish the main quest before your level cap raises.