Make sure it's the correct size belt (I'm assuming you're replacing the old one), compare them side by side, pulled tight. It shouldn't be that hard to slip on. Assuming you're placing it on the correct crank pulley (I'll give you the benefit of the doubt on that one), and that it is the correct size, you may be able to get some more play or slack in the alternator by loosening the lower mounting bolt, not the bolt on the slider, but the one that allows the alternator to pivot... if you've already loosened that (I've always had to in order to get any significant play in the alternator), I'd say you need a bigger belt.