You need to download the Twixtor 4.5 installer and use your Twixtor 4.0 key to install. There is no upgrade fee if you purchased Twixtor 4.0. The installer is on our website, in the Downloads subsection for Twixtor.
Then you will be good to go (make sure to check the "install for FPC on Intel Macintosh" option when installing.
FCP makes it difficult to take projects back and forth between the two systems.
If you are not concerned about older projects you've created, you can install Twixtor for Intel Macs on your PPC Mac (yes, I know that sounds incorrect, but you can do it), and you'll be able to move FCP projects back and forth without having to worry about compatibility. You'll just need to make sure to get rid of the old Twixtor plugin from your /Library/Application Support/Final Cut Pro System Support/Plugins on the old machine, and make sure to use FCP 5.1.4 on both.