I just downloaded the Heart of the Dead mod, and it comes in 4 parts. All of the archives are in a .rar format, but the files in the archives are shown as .7z.001, .7z.002, etc. The first archive opened fine and I extracted the files to my Data folder. but when I try to extract from the other 3 files, it comes up with a "cannot open file as archive" error message! Does anyone know how to fix this???

I had that same problem; eventually I just downloaded the huge, all-in-one file from their main site, since I'm a little impatient and my first few attempts (doing what it said in the readme) failed.