I can't tell you were you are going wrong, but for me, it works by either starting the script from an icon or starting the script from CLI.
I always use Run >NIL: to start it, as I don't want to have 4 open windows telling me the same thing, for the 4 shares I usually mount. If I really need to disconnect one of the drives, I use the break command instead.
I have the following text in one of my script files.
Run >NIL: C:SMBFS Workgroup=MSHOME User=PCGuest Service=//Desktop/Amiga Device=Amiga_SHARE: volume="Amiga on Desktop"
I use StackAttack, so the stack size is taken care of automatically. I use version 1.66 of SMBFS.