I'm not entirely sure what you mean by "It doesn't exist inside the script as it is, but this function is a component of actxcontrol". Possibly, you mean that SC_Enable is a method of the activex control MGMOTOR.MGMotorCtrl.1.
In which case, the call could be rewritten as:
It remains to be seen whether h1 is the same h1 as used by your script. It's never a good idea to use global variables and, not having the parallel toolbox, I'm not sure how it handles global variables. You need to make sure that h1 is actually a reference to your control within the parfor. At the moment, it looks like matlab thinks it's a double.
As I said, I don't have the parallel toolbox, so I don't know how exactly it works, but I assume it creates multiple threads from a parfor. If so, it's unlikely you'll be able to interact with a control from multiple threads. The UI framework of Windows, Linux, etc. (nothing to do with matlab) will only work properly when UI elements are modified from the main thread, not from parallel threads. Attempting to do so may result in a crash.