What program do I need to edit these?

You need RenX or 3DS max with the W3D Import and Export plugin.

Then your able to edit the w3d model files. But you should read some tutorial before you start with modeling, its much harder than dds skinning =)
You can find some tutorials here:
www.renegadehelp.net

Understand that "RenX" is here a Gmax version, not Renegade X the game :p

But yeah modeling is quite hard

Do I just unzip the w3d importer into RenX?

I know it's hard XD but hey I'm willing to learn and practice :3

There should be a readme in the folder. If not maybe someone else can help you.
I dont use and i never used RenX/Gmax.
I use my 3DS max 8, sorry.

But i guess unzip or move the folders inside the zip folder to the correct folders in the RenX/gmax dir.