As Steve has noted, Don Moyers manual is excelent. I recently ordered one and it arrived in only a few days. It is worth the investment. In the meantime you may want to try a few things. First, disconnect the shift cable or linkage and make sure your problem is in the transmission not the cable or linkage. If it is in the transmission, I think the adjustment you need to try is the Reverse Gear one in the instructions Steve posted. While waiting for the manual to arrive you might check the Tech Tips at the Moyer Marine site. The first one is titled the Kaminsky Modification (sounds like Steve learned about his transmission the hard way). It has good pictures of the inside of the transmission. The 3/4" adjusting nut that it refers to is the same adjusting nut that is used to adjust the reverse gear.

By the way, the adjustments on the transmission are very easy, especially the one for reverse gear. Once you figure out what the parts are, there is nothing to it. For reverse gear, all you need is a 3/4" open end wrench. The adjustment for reverse gear is also the one that adjusts the neutral.