You must not have used one of these guides before, and be under mastery rank 20. That's about it.

Go to http://trovelive.trionworlds.com. You should see something like this:
Click sign in, and sign in to the same account you use to play Trove. Click on the apply code tab under account.
Then, just paste the following in: 7JKECHHDCM9PF2TKEDEW