I've heard your feedback, it seems these secret skins are maybe a little too secret..
Secret skins are activated by setting your character's name to one of a few secret names.
To give you a hint, you'll find that the achievement names for each secret skin directly relate to the name needed to activate the skin.
Maybe try setting your name to that of the developer of a game which has just released its bigger final (final) and final boulder update, for now...
