First off, I assume you mean WPA? Make sure your router is set up first, then create a new connection through the control panel on the 770. You may want to try manually configuring the network on the 770 (select 'No' when asked 'Scan for available WLAN networks?'). Manually enter the network name and select 'WPA pre-shared-key', then it will ask you for the WPA key.

If you still have problems, try changing the WPA algorithm on the router. I use AES, but I haven't tried TKIP. The 770 may not support TKIP.

One important note: I don't believe the 770 supports WPA2 (someone please correct me if they know differently).