The file you downloaded should be RWMfor113c.7z. Follow the links, click on download, then click on the third link, RWM for 113c. From there you should find a file named RWMfor113c.7z. This file cannot be opened without having an updated archiver. The creator of this file format was 7zip. If any of that is confusing you please just download and install a program called 7zip and it should open the file no problem.
Once downloaded and opened. Open the folder RWMfor113c. Inside should be a folder called Runewords, a file named 'How to Install.txt' and an image. The Runewords folder should be placed inside the Diablo II install folder. The 'How to install.txt' is the guide that will tell you how to install the RWM. The image is a copy of what your short cut should look like after you update it. The image is in German, I believe, but the places where the info should go is the same and the How to file is in English.
Any more questions I will gladly answer. Please post back if you cannot get it to work.