Mower won't go in 1st or 2nd gear

I have a 2004 MTD riding mower that runs great but won't go in 1st or 2nd gear. I replace the belt just about a week ago. 3rd, 4th, 5th gear runs great but not 1st and 2nd. I tried to adjust it but I can't figure out how to. I have a manual for it, but it doesn't show you much. Please Help!!!

Jared