I'm assuming you have WM03 on your X5 so I suggest the following:
1) Make sure your router is operating in B mode;
2) try disabling the security settings and see if you can connect;
3) with the card inserted, go into the list of networks on your Axim (Start, Settings, Connections (tab), Connections, Advanced (tab), Network Card), hold the stylus on your network and select Remove Settings from the popup (you may need to unplug your router before it will actually remove the settings). Do a soft reset and when the Axim finds the network, chose 'Work' and enter the WEP key in hexdecimal format (10 or 26 characters, exactly).
4) Look at the
Check these first part of the Troubleshooting section of the WiFi Guide for more possibilities.