Its right where the archons are. It's the bottom middle portal of the archon waygate. Once you have killed all 4 archons you can activate it and go right through it. There are I think 3 stages prior to Rag; Frozen General, Grendal, and KillRoy/Andreal. The reason I bring them up is sometimes the process confuses players into thinking they didn't find the Rag mission. When you kill any of those bosses you will be teleported back to your base. So simply just go right back to that portal again to fight the next stage until you eventually reach Rag.
