The installation instructions tell you to directly extract it into the garrysmod directory, not addons. It overwrites a whole bunch of stuff, and I’m afraid if I delete it I might delete something important. I also have a few other addons there, and I don’t want those to get deleted.
Redownload dismemberment mod and look at which files there are. Then delete those files. And then don’t put addons in the main directory for exactly that reason.