When Provisioning tells you it can't find the file you're telling it to run, in this case powershell.exe, its usually leading you down the garden path! It's far more likely that the problem is with the command parameters - the location of the powershell script. My first guess would be the fact that there are spaces in path to your ps1 file and you've not used quotes.
I would suggest running the command manually outside of provisioning first to ensure syntax problems like this aren't going to sneak up and bite you. Also, I can't see if you've enabled powershell script execution. By default this is off so it still wouldn't work.
I've attached a template I've used a few times in the past that faciltates powershell usage, it should give you some pointers ;-)
Steve