Class action suit against Mt. Gox?
Normally class actions are opt out, not opt in.  That means you don't have to do anything if you want to be in the class.  But the decision as to whether it will be what they call "certified" as a class is probably a long way off.  If the court allows the class to be certified, they will then send notice to the class members giving you the chance to opt out of the class if you don't want to be in it.