Hi,
To start with, yes, The Alfa will
work on your Mac.
Do you have the latest drivers from Realtek? I would not try to use the ones from the CD as they are typically quite old.
You do need to "Enable" the adapter on the Mac. This is simply a matter of going into your
Network Prefs page, selecting the Alfa, and clicking on the "Apply" button. It sounds like you already did this though or the Realtek Utility will not show any networks to connect to.
If the Alfa seems to connect, but you are not getting online, check your IP addresses as well. This is also shown on the
Network Prefs page. If the IP address starts with a "169.", it is not a valid IP address and something is still not correct.
This could be an issue on the Access Points side also. Try to use it with another network you know is OK with your
Airport as well.
I can give more specific help if you want to PM me.
Mark