It's all there on your PC. First have a look at the help files for "GPP Tool". The GPP files are the post processor files and are stored under \SolidCAM\GPPtool and comprise of a *.MAC file (initial machine settings) and the *.GPP file (the guts and logic of the post).
There is also a manual in PDF format you can download from the SolidCAM website which goes into a little more detail than the online help.
The language is similar to BASIC but isn't that sophisticated; there is no CASE SELECT statement for example.
Finally there is small but enthusiastic group of post tweakers hanging around this forum who are on the whole a friendly bunch.