Are you talking about the actual dash or the trim around the instrument panel??

For the actual dash:

Note: Do not sand the dash totally smooth unless you plan on painting it.

Depending on the depth of the scratch, use 550 or higher grit auto sand paper and rub the scratch out. Then use an orbital buffer with a foam pad to buff to a shine.

For the trim:
Again use sandpaper and then repaint (as the paint will be sanded off the trim).

Hope this helps?