It's a .dat file?
I've not messed with it, but that sounds like you're suppose to put in in your SH3 folder somewhere and load it up like a Single Mission.
Is there a Readme file or instructions at the place you downloaded it from?
Wish I could be more helpful.
Wait! I can!
Are you sure you're downloading the right thing? Here's a link to the one I JUST downloaded:
http://files.filefront.com/Hydrophon.../fileinfo.html
You'll have to use Win RAR to extract the files, but it has a Readme to tell you how to install it and everything.
Hope this helped!