You could drag your identity folder (~/Documents/Microsoft User Data/Office
2004 Identities/<Your Identity>) to the same location on the other computer,
and then you would have the exact same identity, rules included, on the
other machine.
Unfortunately there is no way to transfer just rules. If you would like to
see this capability in a future version, be sure to send your feedback to
Microsoft. In Entourage X or 2004, go to Help -> Send Feedback on
Entourage. Otherwise, click on this direct link to enter Mac Product
Feedback:
<
http://www.microsoft.com/mac/feedback/suggestion.asp>