1. Set up your 64-bit WEP on your router as normal, and make sure Key 1 is selected.
2. Go to your Mac and enable Airport if its not already done so.
3. From the Airport menu, select the SSID of your network
4. When asked for a password, enter 0x (thats zero-x) followed by your WEP key, i.e. 0xFF12345678
5. Your computer should now connect with no problems at all.