Getting the computers username in to a text box in indesign

Hi

I'm totally new to scripting but at my firm they want to have a text box on their InDesign documents that will show the username of the computer. The idea is that this would be tagged a little like you do in data merge so the box would be '<<USERNAME>>' and this would automatically change to the name of the computer account using it. We did have aguy working on this but he left. I found a script on his computer that had the file 'username.jsx' in his InDesign scripts. When you open it simply has the line '$.getenv('USER')' in it but I have no idea how to make this work.

We are all working on macs.

Any help would be appreciated.

Thanks

Thom