Make sure you are not playing WoW when attempting to copy your character. To actually copy a character, go to the character copy page and follow the instructions.
To use premades, you must have updated your account to the latest version of the game at least 6 weeks before the PTR opened! In order to do useful testing on the Public Test Realm, QA has given us the ability to copy our own characters or to potentially use premade max-level and well-equipped characters to test on the PTR so we don't have to begin a level 1 character and level it instead of testing.
9.1 A simpler way to update character/server name in your WTF folder.