Instead of:
system("actualFile.mpt");
Try:
system("mpt.exe actualFile.mpt");
Or (to be more complete):
CString command;
command.Format("mpt.exe \"%s\"", mptFile);
system(command.c_str());
If mpt.exe is in your path, then that should actually invoke mpt.exe.
The only question then is if the mpt.exe program actually parses it's command line and opens files on the command line, or if it expects some sort of command line switch. (Such as "mpt.exe /o filename.mpt" or something similar.) If you don't have documentation on what mpt.exe expects on it's command line, you can always experiment with variations typed in from the command window and see if you can hit on something that works.