i'm kinda newbie too so all i'm going to say might be wrong...
ok, firstly, telnet is NOT secure. try not to use it.
use OpenSSH instead.
if you don't have OpenSSH, you can download it at www.openssh.com
if you still want to use telnet, here are the possibilities i can think of:
1.) you need to have super user rights to use telnet on your PC. in this case you can either login as superuser(bad!) and use telnet
set the permissions of the telnet service to allow your normal user account to access it.
2.) the server you are trying to telnet to allows only its superuser to access it through this remote connection. in this case, u gotta be the superuser of that server...
3.) i might just be wrong, wait for the experts to come reply to your post
hope i've been of some help though!