Here are the locations of the three disco areas:
1) There is a broken disco ball in the Episode 2 room of the Cantina. It is easy to spot as it is in the center of the room on a table, so just use a Force-prone character to fix it.
2) There is a disco room in Episode II, Chapter 2: Discovery on Kamino. You CANNOT access this room in Story Mode, as the room has restricted access to Clone Soldiers/Imperials, so you will have to use Free Play Mode. The room is located in the same hallway as Jango Fett's room on the left a little further down. After some set-up and stepping on some lights on the floor in as they appear, you'll set off the disco.
3) Finally, there is a disco room in Episode VI, Chapter 2: The Great Pit of Carkoon. You CAN access this room via Story Mode and isn't restricted by any doors requiring certain characters or other barriers. It will be somewhere inside Jabba's barge, and it requires of bit of set-up for speakers and getting all the pieces in place for the disco ball, not to mention doing the same thing as in the one on Kamino where you must step on lights on the floor. Once this is all done however, the disco will activate and you'll hear that sweet sound of an achievement unlocking.