Download the archive here interstate_82_complete.7z (3.15GB)
Use Winrar or 7zip (both free - links below) and extract directly to your Battlefield Mods Folder
By default this is:
C:\Program Files\EA GAMES\Battlefield 1942\Mods
or
C:\Program Files (x86)\EA GAMES\Battlefield 1942\Mods
This folder should now look like this:
Play!
WINRAR -http://www.rarlab.com/download.htm
7ZIP - http://www.7-zip.org/download.html