I second the fact that some tires and rims are just really tight and sometimes you have to use the tool to get it on.
Here's another tip. Once you get the tire mounted almost all the way, check to make sure the tube is not being pinched and then go all the way around the tire pushing the bottom of the tire in towards the rim; this is kind of hard to describe. But, if you do this, it helps get a tight tire on.