To switch channels, you have to wait for a teamspeak admin to help you and give you registration rights. (Not a SA:MP admin).
The teamspeak admins have "SA" written near their names.
Example:

Please note that I am
NOT a teamspeak admin on the Argonath RPG teamspeak server.
The letters near a name specify the status of the user.
R = Registered
U = Unregistered
SA = Server Admin
CA = Channel Admin
Cheers and I hope I helped,
-xcasio

"The purpose of life is to live it, to taste experience to the utmost, to reach out eagerly and without fear for newer and richer experience."
- Eleanor Roosevelt