When you get the first login screen it will be for superadmin. Type superadmin into the username field and type your superadmin password into the password field. Click Login not admin/client login. If you changed the passwords when you first installed TS Server you need to exit by right clicking the Task Bar icon and selecting exit and then restart it by clicking the TS Server icon on your desktop.
If you still get the error uninstall Team Speak server from Add/Remove Programs, then reinstall it. Make sure you have the latest version. You can get here:
TeamSpeak - Downloads
You need the TeamSpeak 2 Server - it's the 3rd item in the list.
After you've installed it - it will give you your new superadmin and admin passwords. Try logging in again.
If you still get the 403 error - please post back what web browser you are using and also what operating system you are using.