@joeller and anyone else running into trouble here
- make sure stsadm.exe is in your PATH
- even with stsadm.exe in my PATH, I had an error with the given command on Windows 7 … surrounding the filename with quotes does the trick for me:

stsadm.exe –o addsolution –filename “WSPBuilderDemo.wsp”

joeller :
The command line command “stsadm –o addsolution –filename WSPBuilderDemo.wsp” merely produces the error “command line error”, then lists all the possible operations for the command. “stsadm.exe –o addsolution –filename WSPBuilderDemo.wsp” and “stsadm.exe -help” produce the same thing.

This is the correct syntax. Please see the following technet page: http://technet.microsoft.com/en-us/library/cc288721(office.12).aspx – Is your WSP file name WSPBuilderDemo.wsp?