The Milwaukee Wedding Hall is the place where Gordon and Jodi (which is now Mr. and Mrs. Birch) celebrate their wedding in the episode iDo.
The iCarly gang and Spencer go to this place (because they were invited to the wedding) where a lot of trouble occurs due to a misunderstanding between Gordon and Jodi along with the other guests, which is immediately solved.
This is also the place where Carly performs "Shakespeare", which she dedicates to Jodi (as Gordon is too afraid to sing it due to stage fright).