Regular users of ssh.uio.no are encouraged to download an SSH client. A native client is faster and has better functionality than what this web application can offer.
You can also check out the PuTTY documentation.
When you start PuTTY you only need to fill in "ssh.uio.no" (or another unix-host) in the host-field and press "OK", you will be prompted about username and password.
Newer versions of MacOS has SSH built in; start a terminal window and type "ssh ssh.uio.no". (If you have a different username on your Mac add the option "-l your_UoO_username_here".)
Earlier telnet was the usual thing to use when you wanted to log onto a UNIX machine. The snag with connections of this type is a low level of security, among other things username and password is transmitted as plain text over the network. Today one should use Secure Shell (SSH) who addresses many of these security issues.