Q: 12" powerbook G4 upgrade hard drive

You need to expand on "having trouble".

The immediate questions that come to mind are:

What are your PowerBook's specs and do they meet the requirements for Leopard?

http://support.apple.com/kb/HT3759

Have you formatted the new hard drive - MacOS Extended?

Sorry if these questions are a bit insulting but we have to start somewhere!

Its probably also worth having a quick look at this:

http://support.apple.com/kb/HT1544

They are not insulting, he/she asks for help without giving any info regarding model, RAM installed, what he/she tried to do etc.

Are you using a retail Leopard install DVD. It should look like this.

You can't use a gray installer from another model Mac.

 Cheers, Tom