If you're searching for a cheap yet MacOS X proof Serial (DB9 / DB25) to USB adaptor, I suggest that you look at the Dacomex one. If you already got one of them, you will have to download the driver from the internal chip manufacturer.
This driver may be used for other adaptors... so if you tried the pl2303 driver and that it didn't work, you should try to connect it to a GNU/Linux box and check the internal chip using lsusb. The FTDI one appears as Future Technology Devices International.
On MacOSX, you can also use Zterm, a Terminal.app clone for serial devices.