As you probably guessed, there is an ad for Toy Story 3 in Poptropica.  To complete the ad, you must rescue Hamm, Mr. Potato Head, and Rex from the cages that Lotso has locked them in.  You must get to the cages and unlock them without getting caught by Lotso.  To do that, you must avoid the beams of light that are produced by the security cameras.  Hide behind things like glue, chairs, and other objects so you are not caught in the light.

The prizes you get are either a Jessie costume or a Woody costume (depending on whether you are a boy or a girl) and an Alien follower.