The following NSXs were sold in North America with power steering:

1991-1994 automatic transmission NSX
1995-2004 all NSX except 1999 Zanardi NSX

The following NSXs were sold in North America without power steering:

1991-1994 manual transmission NSX
1999 Zanardi NSX

It was never an option. Depending on which model you bought, as noted above, it either had it, or it didn't.

Incidentally, the manual steering on the NSX is really wonderful, and gives a great feel. It's easy to use, and even slow-speed parking maneuvers aren't too bad.