You cannot redeem WorldPerks miles for car rentals.

The best you could do would be to put them towards a WorldVacations package (which might include a car rental as part of the package). NW has very limited redemption options beyond flight awards and it is one of the larger holes in the program as far as I'm concerned. You can explore the non-flight award options here:

http://www.nwa.com/worldperks/travel/