How do I specify what program to use in a new item if I don't know the path to it?

If you don't know how the exact path to the program you want you can use the "Select..." button on the creation window to display a finder open panel. The Finder and system dialog boxes, such as the open panel, hide the UNIX file system paths, such as /usr. However you can use the "Go to:" field in the dialog to jump to /usr, or any other UNIX path. If the program you want is somewhere in /usr just type that path into the "Go to:" field and click the "Open" button to see what is in /usr, from there you can navagate to any sub folder you need to get to the program you want.

The open panel's "Go to:" field allows for quick access to UNIX paths.