I am also running Leopard.
It was a VERY easy install. Download the package, run the package, enter your password, confirm that you want to install, restart computer, turn on droid, go to droid settings>application>development>USB Debug then go to PDAnet start the USB Tethering and cick the icon at the top of your menubar on the mac.
This was also typed on my macbook using the tethering.
Running 10.5.7 OSX. Downloaded pdanet 2.15.
Begins the installation process. Right at the last moment
it tells me that it failed and to contact manufacturer. Driving me up the wall!