Andy,
I am not into plasma, but will try to help a bit.
You use a cad program to make the drawing of what you want to cut. (I use Draftsight)
Save the drawing as a DXF file (I use Rel 14 ASCII format)
You then open Sheet cam and import the drawing and use that to define the operations. Depending on what your doing, you may want different operations on different layer of the drawing.
In the options, you select the correct post processor and yes, there are several torchmate post processors. The correct one for your control software will generate the appropriate G-code to run on the machine.
Control software. There are several software control products on the market and Mach is just one of them. Another one is linux based Linuxcnc and there are several more. A quick glance on the torchmate website seems to indicate their Accumove motion controller is another one and probably the post processors listed in Sheetcam are for it.

As I say, I am not into plasma, but the above dialogue is similar regardless of machine type be it plasma, router, mill, lathe etc.

Hope this helps.