Zumba Fitness

Just as the achievement description says... simply join and complete any Online Class (even if it is on the last routine - it still counts! - you can see the progress through the class as a % in the last column when you search for online games.) The achievement will pop after completion of the routine.

(Edit) Your name needs to appear in the score roundup at the end of the routine for the achievement to pop. Sometimes this game looses track of your save file for some reason. I strongly suggest moving your save file to a different storage device and loading it before playing online. For whatever reason, this seems to help keep your name appearing at the end of the rounds. Also - if you finish signed in as 'Guest' which happens more often when you leave view of the Kinect or use the Guide button during play - it seems to not list your name... which means it doesn't count as a completed routine AND any new tags you might have danced with did not count!