He might need a little more help than just a bunch of acronyms thrown at him.
7-Zip is a program that compresses and un-compresses (not called decompress) files into what is called an archive file. It's like putting a bunch of paper files you have into a bag and then vacuuming out all the air to make it take up less physical space. You can get it here
http://www.7-zip.org/, it's free and is probably the very best of the archiving/compression programs out there.
As arizonachris has stated, most downloaded files from web sites are either in .RAR or .ZIP formats. 7-Zip handles them all (that I know of). Inside most of them are a file that ends in .RWP or lower case version of it. That is the main file you are interested in. It is installed using the launcher for RailWorks. Some of those files come with an executable file ending in .EXE or .exe. It's generally not safe to just run any old exe file from someone you don't know. Viruses can hide inside exe files and when you execute them, the virus (if there is one) can infect your computer with that virus. Just trying to educate a little here, not scare the crap out of you. Most files you run will be virus free. Just saying.
The other way and simplest way of getting DLC (download content) is by buying it from Steam. It will install automatically and be kept up automatically for you from Steam.
Once you get the DLC into your sim, you sometimes will have to manually add the content like rolling stock to some of the scenarios you want them in. Because the DLC didn't come with a scenario that utilizes the rolling stock for instance.
Say you want to add some box cars you downloaded from this site on Castle Rock scenario... You will first have to "un-lock" the Castle Rock route. Here is what I do:
1. From main menu in the game click editor.
2. Click scenario editor.
3. (Assuming it is locked) click create new scenario.
4. Click OK and then give it any name.
5. Once in the editor mode, click the lock icon at the bottom right to unlock it. You get a warning, click OK.
6. Click the play icon. (looks like a triangle point to the right)
7. Click no for saving. You don't need to save it. This so far was done just to unlock the route. Crazy, I know.
Now that your route is unlocked you can go back to the main menu then back to the scenario editor and edit any scenario on that route. But if you use Steam, then you should clone the scenario you want to edit first and edit the cloned scenario. Because Steam will fix your original scenarios back to factory default eventually. And re-lock the route from editing.
Hope I helped a little more than I confused you. Good luck and have a happy Thanksgiving.