I Assume user and pass refer to the proxy credentials, what does username and password refer to?

These refer to the basic authentication(as per RFC2617), not the proxy authentication. I assume that you prepend the auth credentials to the proxy URI as demonstrated in RFC2396 §3.2.2, though it is said to be "not recommended".

if my proxy were a hostname rather than a web address would I just insert http:// before the hostname, or use the IP address, or just the hostname?

Hostnames should be resolved to their corresponding addresses; see your network administrator and/or consult your operating system manuals to find out how to set this up. The docs do always make an indication of an initial http://
It sounds like you already have it set up, though, so http://proxy-hostname:port ought to be sufficient. Note that this does not work for anything besides HTTP proxies; SOCKS will not work.