It's annoying that you can't get to this in the properties. The easiest way to get to the URL would probably be to export the RSS feeds to an OPML file and you can find them all in there.
I'm using 2010, but I think in 2007 you start from File > Import and Export (in 2010 it is now in File > Options > Advanced > Export). One of the export options is to export your RSS feeds to an OPML file. You can do this for all at once (which is handy for transferring to another machine, or backing up), or just the specific ones you are interested in. Choose a filename and location you can remember. Open the OPML file from a text editor such as Notepad and you can see the URL for the feed XML