Linksys WKUSB54GS cannot associate with access point problem

I just bought Linksys WKUSB54GS (Wireless-G USB Kit with SpeedBooster),
which includes

WUSB54GS 802.11g Wireless USB Network Adapter With Speedbooster
WRT54GS Wireless-G Broadband Router with SpeedBooster

I thought Linksys brand should be good router and adapter. However, it
always have error "cannot associate with the access point" or
"associates with access point, cannot find internet" error messages.

Do you have some ideas whats going on? Should I change another wireless
router? Any suggestions? or good wireless router/adapter?
please advise.

thanks!!