Hi feajob.
In your installation directory, in a subdirectory called /conf, there should be a file called nast200x.rcf (x stands for your NASTRAN version). Test-edit it and add a line with your scratch directory complete path:
sdir=/your_complete_path_to.../scratch.
It should work.
Regards,
Spirit
'Ability is 10% inspiration and 90% perspiration.'