I use a Belkin usb adapter with a driver (RT2571) available from Ralink technology. See
http://www.ralinktech.com/ralink/Hom...Macintosh.html
The first 2 are for USB. (RT2500, which includes 257*) and RT2671 )
RT28** is a newer driver which also works on Leopard.
If you can find the name of the Chipset used in the usb adapter, (sometimes named on the box or in the manuals) you may be able to search for a mac driver which works with one of the above chipsets.
The following list gives some brands of usb keys which use chipsets which work with RALINK drivers:
http://ralink.rapla.net/ See the USB column
Other brands will use other chipsets. These other chipsets may also have downloadable drivers for mac, but you'll have to search for them. I recommend the Ralink drivers.
JB