VERIFY the IP address of the Westell 6100 without the Linksys connected. I've ALWAYS seen the factory settings of 192.168.1.1. The Linksys router (WRT54G/GS) defaults to 192.168.1.1 too. That's the problem, I'll bet.
If this is true, you must change the MODE of the Westell to BRIDGE, not...