if you've rounded the allen screw for the screw (not the wrench), then use a small dremel cut-off disc to put a slot on the screw head, then try using a precision regular screwdriver to back it out.
Alot of those screws have a lock tight compound on them, and they are hard to get out.
If that doesn't work, drill a smaller hole, rethread it with a left hand thread, screw (with left handed threads) in another screw with a hex head on it, and use a wrench to back it out. When you tighten the screw with left handed threads, it will forse the orginal screw to thread counter-clockwise, backing it out.