Unfortunately, using only MAC filtering on your WiFi network won't keep out anyone but the most ham-fisted hacker. Even the basic "live" Linux CDs have the ability to sniff WiFi traffic...which will show the MAC addresses of clients accessing your AP. All that needs to be done at that point is to provide a "locally administered" MAC address (to the hacker's WiFi card) that matches an approved MAC on your AP...one they've seen exchanging traffic with your AP...and they're right back in business. Simple WPA encryption is a virtual necessity.

dougef, check to make sure that you've set up the WiFi security in the powerline bridge itself. Think of it as a completely separate AP...because that's basically what it is. Check your manual for how to get in and configure it...then try cloning the same WEP/WPA, SSID and channel settings from your main AP.

EDIT

Sorry SB, I was typing my response while you were submitting yours.