Gmail Notifier Plist Problem

Hi all.
I wonder if anyone can help me here.
I've made a Gmail Notifier as documented here (i'll send it on a reply, no urls for first post!)
I'm using a mac with osx 10.4.11. When i run the python script in terminal it all works perfectly. However, when i use launchd to open the plist, terminal has a problem with the python script, namely 'ImportError: No module named serial'. This is obviously not the case as the script works fine when run manually, its just when run by launchd and the plist that this problem occurs.
Any ideas?

Here's the actual tutorial i followed