Note the first line in the script:
(*WRITTEN by ZAPWIZARD
Requires GPSNavX available from here:
http://osaxen.com/files/serialportx1.0.html
and XML RPC*)
You need to install the serialportx1.0.html and also the XML RPC scripting additions before this script will work. They are easy and free, but required.
If you did that, then you are getting a driver error. But it looks like in trying to execute the command to get a list of the serial ports -which depends on the serialportX scripting addition to do this on Applescript's behalf, it is getting an error. That points to the scripting addition not being installed.
One way to test your GPS is to download GPS Utility. You can set your serial port and watch the data come in from the GPS. If that's working, then you know your GPS is working.
Also, GPS receivers aren't $200. You can get one of the BU-303's for under $100, I'm sure.