Sounds to me that you are accessing the DSL modem instead of the WRT54G router.
Try disconnecting the cable from WRT54G router to the Verizon modem.
If need be, disconnect all cables except the one between the desktop and the WRT54G router.
Download a users manual from Linksys for all the settings.
You may want to consider changing the Router IP address while you are in there. Middle section, first page.
There may be problems If the router and modem have the same IP address.
All you need to change is the third set of numbers 192.168.xxx.1
Then your computers need addresses in this range.
Restarting the computers should allow them to pick up a correct IP address.
Enable WPA on the wireless and set a key to secure.