those asa screws can be a pain to get out if they heads stripped. if it feels like the allen key isnt setting into place and turning anything it might very well be stripped.
1. double check you are using the correct size, just in case.
.....if you use the wrong size, well, thats usually how they get stripped.
2. so far the best way to get one of those out ive found, is to find an allen key that fits the closest, and glue it in. let it set for a long time to make sure its definitely dry, the very carefully try to turn it without breaking the glue loose. since those asa screws go down pretty far, it might be easier to just turn it further inwards instead of out, so that way you can grab the bottom of the screw with a set of pliers and turn it the rest of the way out.
3. you *might* be able to get another screw, maybe the one from the other hole and put it in on top of it, maybe turning that one on top of the stripped one will grab enough to turn it the rest of the way out. be easy on this one though, if it doesnt turn it dont keep cranking on it, the threads in the frame (which is aluminum) will give way before the ones on the hardened steel screws.
4. try putting some penetrating oil in the hole and let it sit for a couple hours. it might be enough to break it loose from the rust/gunk that can get in there and at least make it easier to turn.
5. if you absolutely cant coax it out. your going to need a drill press, the correct size drill bit, level, and probably a heli-coil or thread tap afterwards. (aka just drill the bastard out and tap threads for a size bigger).
6. if the other screw is fine, it should be more than enough to hold the asa in place. just tighten it down and go play. most rail/asa combo's only have 1 set screw or ball holding them in place anyway.
7. if you can get it out, just replace it. any hardware store should have it. double check with the manual, but it should be a 1/8th" grub screw. 10x32 threads. should be about 50 cents.
edit: you can also try hammering a size bigger allen key into it, might be hard due to where its at, but its one way. probably going to ruin the allen key this way though, so make sure its not part of your "good set" lol.