One can not use the portals to go to the deep dark realm. Could this be looked into or is it blocked on purpose? Thanks
As Tyler said you must use the MyM portal to go to the Deep Dark dimension. (Build a Nether portal and place a sign below the obsidian with text: portal on the first line, then power the sign with redstone and you should see a GUI then click on the Deep Dark to unlock it and there you go. + info here)