This adapter is based on the ZyDAS ZD1211 chipset.
Mac OS X Drivers
ZyDAS has been bought by Atheros, and any official support or development on the Mac OS X drivers was halted.
Nevertheless, you can get the drivers here.
I can confirm them to be working OK in PowerPC machines running 10.4.9, since my mini is currently hooked up via one.
Hacking an External Antenna
Although the RF circuitry isn’t designed to handle a 2dB antenna, you can get a significant range boost via a very simple hack – just take 1.5cm (or so) of solid copper wire to form a little rod and solder it to the PCB antenna on the designated spot:
Make absolutely sure that you have enough of a connector surface to lend mechanical resistance to the join – otherwise, a lateral pull on the copper rod will tear the PCB along with the solder.
Finally, cut a small hole on the casing to let the wire protrude:
If you picked the right gauge wire, the antenna connector will grip the wire tightly enough to hold the whole assembly together:
Nevertheless, you might want to consider dabbing the connector with one or two drops of glue (with a glue gun) to make sure there is enough mechanical resistance to avoid tearing the PCB – especially since the weight of the antenna connector alone is far greater than the SMCWUSB-G’s.