if you are using a 64 bit version of Qlikview, then you should use the config_x64.xml. If not, the config.xml file you are looking for should be in:
C:\Documents and Settings\All Users\Application Data\QlikTech\QvWebServer.
I'm not sure about the URL part but i believe you can put it in the beginning right after the <config> section starts. This guess is based on the observation that there is a <ConfigUrl> section and a <TunnelUrl> section in the 6th or 7th line so I would assume that's where any other <URL> section would go.
Hope this helps