The Dark Portal is in the SE corner of the Blasted Lands, surrounded by level 61 elite demons that hit you for 2000 damage per swing.
I had to corpse drag myself to get right up to it, but you can see it from a safe distance. Currently, it goes no where, but I think the expansion will have something to do with it. Blizzard said long ago that they were going to have an event that would require lots of level 60 warlocks to go to the portal and open it before anyone from the realm could go through.
They probably lied, though.
