Have you updated the Airport software on the Mac? If the USB adapter is using the Broadcom chipset like the Airport Extreme card uses, then the Airport software should pick it up as a "third party Airport device" and you should be able to set it up as if it were an Airport Extreme card.
While not a USB adapter, I've done this with a Bufallo Technologies PCI wifi card that uses the same Broadcom chipset and it worked flawlessly without any extra software other than the latest Airport Software.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1