Make sure you setup a connection in your 3DS internet settings and name it attwifi. Then test the connection to verify.
You only need to add the MAC address of your 3DS to the Access Control list; not the MAC addresses of the Streetpasses. I need to update that info in the guide. You can even disable the Access Control list to allow any device to use your USB hotspot.
I've been trying to get the XP way to work and well, I've run aground.
I've gotten the WiFi Adapter up to the point where it does act now as a regular SoftAP point and added the MACs to the Access Control (don't know why), however the 3DS doesn't seem to be picking up any of the MAC addresses. I changed the MAC in Techitum to one that's also in the Access Control and it says I have a Nintendo Zone available BUT... it doesn't seem to get a pass. I've tried it with a few of the more populated MACs too.
So I dunno what's going on. The 3DS won't connect to it as a regular WiFi point also despite other devices being able to.