I bought the Asus USB-N13 adapter, because it has the OSX driver. Don’t use the Asus driver, I downloaded the driver from the Ralink website.
Use the first driver for the RT2870. It will install a utility that will connect you to your wireless network.
At first, I couldn’t get it to recognize the adapter, so I tested it on my Win 7 machine. It worked fine there, then I plugged it back into my XH61 and it worked, weird.
There are other Wireless USB adapters that work also, as long as it has the Ralink chip, as Ralink has all the OSX drivers you can download from their website.