the Rogue Wave is what many people are recommending these days.

http://www.wavewifi.com/rogue-wave.html

I haven't tried it myself, but as it is an ethernet device, it should have no compatibility problems. Reports on it are good. The issue with ethernet is you must use PoEnet, which is another power supply and another part to keep track of.

I have been using the WaveRV (actually the previous model that looks the same):

http://www.radiolabs.com/products/wireless/marine-wifi-antenna.php

It says incompatible with Mac OSX 10.6 on some revisions, and it didn't work for a while (driver would not load) but now on 10.6.7 it seems to work again. I would not expect great customer service from RadioLabs. They shrug and say "ahhhhh, I dunno...." But it is a single plug USB device, nice and simple hardware wise.

I guess if I was buying one fresh I would try the Rogue-wave.

Or you could try downloading the Mac driver for your Alpha from the net:

http://www.alfa.com.tw/in/front/bin/ptlist.phtml?Category=105397