I did the prove your self mission and the person I was playing with turned it in and I got the achievement and the still point. Now I cant look at the board for the other missions in the DLC. I joined someone game that had not turned it in, he turned it in and still having this problem. anyway of fixing this? 

It's not another mission, the curtain at the back opens up with three new portals for the longer challenges.

If the curtain isn't open, it's possible that it was done in a different playthrough.  Maybe you joined the other guys game in playthrough 1, but you went in in playthrough 2