Afraid a USB-Parallel Port adapter will not work, it does not have the strict timing constraints that are required for Mach.
If you do not have a parallel port on your computer then you can go down two routes, one is to get a PCI addon port, the other is to get one of the many external motion controllers available for use with Mach.
One other thing worth mentioning, the parallel port will only work with a 32bit operating system, so if yours is 64bit then you will need an external controller.
Hood