EZSkin 3 PDF Instructions specifically states that although scripts prefer to be placed in the main Poser runtime, EZSkin 3, in particular, can be run just about anywhere. I recently set up a 'new' computer with a fresh Poser 11 install and EZSkin 3 directly within the main runtime. Running the script several times has not triggered any errors on my system. Windows 10 is the OS. Now, I noticed a mention of Poser 10 using EZSkin 3; wasn't EZSkin 3 developed for Poser 2014? If using Poser 10, shouldn't you be using the original EZSkin script, not the latest iteration?

As for Poser searching for scripts located in other runtimes, you should use the run script feature under Files, not from the Contextual menu under scripts.