Grand Master Tobias:
To become a Dark Wizard, the appropriate level and token are required. Your level must be at least 20 to qualify to change profession. You must also have a token that proves that you passed the Test of the Dark Wizard.
However, it seems your level is not high enough, and you have not passed the Test of the Dark Wizard, either. First, raise your level to 20 or higher. To take the test, look for Shamaness Varika in the Dark Forest. To change profession, you must pass the test she gives and bring back the jewel of darkness as a token. She is staying at the altar of the Dark Forest. The altar is in the southern part of the forest.