How do you get on the balloons?

Answer 1

The Poptropica Blimp

is always parked above Main Street on the island you are playing. There are only two exceptions : on Time Tangled island, you must select LAB (12 o'clock) on the time device to return to the present. On Astro Knights Island, you must return to the planet Poptropica, where your spaceship will park on top of the castle.

The Balloons on Counterfeit Island

are in the Clown Store. They allow you to jump farther, but unfortunately there is little use for it on this particular island (compared to others).