did you let the tensioners pull all of the belt in? I remember this problem when i installed some springs in like 2003, and i just did some full Tein coilovers...and purposely didn't disconnect the tensioners from their posts (not sure if you can do that on an MSP), and just worked under the belt as best as possible...
I had to fight with it for a while...from what i remember you had to tilt the tensioner back, so the opening slot would be facing upwards...then pull the belt very gently and slowly out parallel to the ground...if that makes any sense...I know it involved something like that, you couldn't simply pull the belt slack 'straight' out of it for whatever reason...but that only happens when all of the belt was gobbled up...
I was only getting a few inches at a time...just stay on it, and don't let it eat the rest back in when it snags...just let a little back into the tensioner, and then pull a little more out and keep going in motions like that...it took me like an 30 minutes for the one side, but after i figured it out i did the other almost instantly...
hope that helps...