Installation instructions say it needs to be in the same folder with the Poser.exe file.
That's if you are putting it in a runtime, you want to make sure it's in the runtime with the actual program. But. . . you can run python scripts from pretty much anywhere. There are some scripts that have other files that go along with them, that all have to live in the same folder together, EZSkin 3 isn't actually one of them.
When I'm working on a script, I often keep it on my desktop and use File/Run Python Script then select the script file. I just tried to make sure this worked. I put EZSkin3 on my desktop and was able to run it just fine from there.