I assume you're logging out on each PC each time you try a different configuration in FreeNAS? Windows caches shares so you have to force it to forget what it saw before.
So, just to be clear, you browse to the NAS in Windows Explorer | Network and you see something like this:
Then you double-click it and you see something like this:
but when you enter the triple-checked user name and password that you configured in FreeNAS for the owner of the share, it doesn't open?
If so, I'm also at a loss, sorry.