Sounds like your DHCP settings on the router need checking.
I'm not familiar with this router, but it looks like it is only set up to allow one wireless station at a time. You need to configure DHCP on the router to allow the additional device to connect at the same time.
I would also strongly recommend that you implement WPA-PSK security on your wireless network, unless you are prepared to allow your WLAN to become an internet hotspot!
Baldbloke, August 2007