the is the site that sells proxy plus. the reason you cant find the admin page is because they are assuming you have the proxy server installed and are trying to set it up.

i dont know of any publicly available proxys that support telnet intentionally. which is what i think your asking for. these require time and patience to find and you'll not find too many people willing to make their find public and loose it.

if im wrong about this you can find an easy to use proxy, for download, that supports telnet at anolog X.