Using the information provided in your welcome letter, login to your Ventrilo server's Rcon:
1) Connect to the server.
2) Right click in the Ventrilo window.
3) Select Server Admin.
4) Select Login.
5) Enter global server administration password.
After you've logged in with the password (using the directions above), you should notice the server name change and have an "A" in front where there was none before. If that happens, you're now logged it with Admin rights. There is no other notification that you've successfully logged in.
After you've logged in, you can edit the Server Comment:
1) Right click in the Ventrilo window.
2) Select Server Admin.
3) Select Server Properties
4) Edit the Comment
5) Click Send to commit the change.