How to Set Up Proxy?

I have installed TikiWiki 3.3 in RHEL 4.
Everything works are successfull excepted only Profiles Repository updates. When I click 'list' buton at Profiles Admin pages, the 'Data Channels' are not updated. I think it related with proxy setting.

The newly installed TikiWiki is setted up inside network of proxy server. That is the TikiWiki be affected on proxy.

My question is that how to set up proxy information within Tiki to successful communicate with 'http://profiles.tiki.org/profiles'?