Strange that the website doesn't state Mac compatibility. As far as I know, all LaCie products are compatible with the Mac. And if you download the manual for that portable USB DVD drive, it does state the requirements for Mac users:
FireWire (version)
FireWire 400 equipped computer
Mac OS X 10.3.9
Power PC G4 or higher, Mac Intel processor
256MB RAM
USB (version)
USB equipped computer
Mac OS X 10.3.9
Power PC G4 or higher, Mac Intel processor
256MB RAM
But I think only the FireWire version comes with extra Toast for Mac software bundled. The older model may still be available cheaper in store and matches your PowerBook better:
http://www.lacie.com/download/datash..._ls_usb_en.pdf
http://www.lacie.com/download/manual...dvd_usb_en.pdf
But this one really needs two USB ports, it seems, while with the new version (Sam Hecht) it is optional when the USB bus doesn't provide sufficient power:
http://www.lacie.com/download/manual..._dvd_sh_en.pdf
You could try and find the Porsche FireWire version, which will definitely require only one cable because FireWire should provide ample power.