The holes in the monster mats should be round for a 2000. There should be nubby little posts on the floor of the car. They should line up with the holes and then you push down on the part with the hole in it until it clicks in place. Perhaps you have the wrong inserts in the mats or are missing nubby posts.
Have you taken the upper part of the snap off the old mats and put it on the Monster Mats.
Forgive me if I'm being to much of a Captain Obvious, but that's the only way I can think of a mat swap as being a problem. If you've never really looked close at the snaps,you might not notice the top piece comes off the mat and isn't part of the mat.
Captain Obvious was right--the top part of the snap was still on the post and that was the problem. I had tried to get them off before, but they were kinda gooed to the carpet b/c the previous owner was a slob and didn't take care of my poor car. I had to get needle-nosed pliers and pry them off of the posts.
And for those of you who were wondering...No, I'm not a blonde, I just play one in real life.