Accessing telnet in a unix shell account is quite simple. At the unix prompt (often $ or your username$) type telnet, the prompt should then change to telnet>
At this prompt you want to type op (For open) and then the address valhalla.com 4242 (see note*)
The following would be the complete line you need type at the telnet> prompt (all but the '*' do not type that)
op valhalla.com 4242*

Another way to use telnet from the Unix prompt is to simply type the command telnet with the address following.
telnet valhalla.com 4242*

If you are using X-Windows, I should think you are capable, simply open up a shell window and then follow the steps in the Unix / Shell tutorial.