Each session has a set number of fitness points associated with stars (similar to points and stars in any Guitar Hero/Rock Band game). As you play, you will continue to see individual drills fill up their own star meter. This is not the same thing. The session rating is your overall points for the entire workout (all of the drills combined). You need to earn 1 star in this rating.
Basically, you just need to actually try in each drill to obtain your first star for the session. This will pop probably before your second drill assuming you 3-5 star your first two drills in a 10 minute workout. I 5 starred my first drill during a 10 minute workout and the achievement popped as I already had enough fitness points.