1 MacBook connects to Westell router, no internet; other MacBook is fine

We have Verizon DSL, and use the Westell Versalink DSL Modem with built-in router that they provide. My sister and I both have MacBooks. My sister's connects to the router's wireless network using Airport with no problems whatsoever. When I try to connect my MacBook to the same router, again using Airport, it says that it is connected, but gets no internet.

I'm aware of the many problems associated with Apple computers and the Westell VersaLink DSL Modems/Routers provided by Verizon.

I tried updating the router's firmware, as one post suggested, but this changed nothing.

I connected my MacBook to the internet successfully using an ethernet cable earlier.

I've rebooted both the router (and left it off for 5 min before turning it on again) and the MacBook to no avail.

When I go into the Network System Preferences on my sister's MacBook, it says she connects using DHCP. When I try connecting using DHCP on MY MacBook, however, while I do get an IP Address and Subnet Mask, the "Router:" field is left totally blank.

When I check what is connected to the wireless network on 192.168.1.1, only my sister's MacBook appears, even when my MacBook's Airport says its connected to the router.

Does anyone know what I should do?

Thanks