To find the grades on his site go to Search and "Grades" scroll down until you see an article like Texas Rangers Top 20 Prospects or Philadelphia Phillies Top 20 Prospects continue to scroll through the pages to find each team. For instance here's the Twins top 20:
http://www.minorleagueball.com/story.../16/1261/65162
Additionally you can just search a team name like Oakland Athletics, and find the article Oakland Athletics Top 20 Prospects like here:
http://www.minorleagueball.com/story.../14/131658/119
Of course the grades are year old, that's why I recommend his book