Linksys Wireless Router :: WRT54GS Will Not Allow To Set Up Encryption

I'm having a very difficult time getting my WRT54GS to work with my Westell 6100 modem. I've been bouncing around gathering info, and what I've learned is that the goofy way CenturyLink tech support told me to hook up my old wired router (modem line into outgoing (LAN) router port) is because the Westell 6100 is a "smart modem/router". Therefore, when I run the Linksys setup CD, it sees the modem as a router, and won't properly configure. I've also learned that the Westell modem can be "dumbed down" into simply a modem by putting it into "bridge mode", which is where I'm at right now - trying to get it reset to factory defaults and resetting the connection type from PPPoE to Bridge. In the meantime, I also reset the WRT54GS to factory defaults, and now I've got another problem: the router will not allow me to set up encryption. The default setting is "Disabled", and when I click the down arrow and try to make any selection at all (preferred selection is WPA2 Personal), it reverts to Disabled. I've also tried to change the router's IP address to 192.168.2.1 (and alternately to 192.168.1.253), so it won't conflict with the modem's IP, and it freezes up every time I try.