Nikon support said that the latest version of Nikon Transfer is supported up to 10.5.8.  There is no official support for Snow Leopard at this point.

There are three things that you can do in this situation.
1. You can partition your hard drive and have a minimal 10.5.8 installation that you could use to import the photos.  Then you can restart your computer and boot back into 10.6
2.  You can wait.  According to Nikon they plan on supporting all USB devices to Snow Leopard.  See here:
http://help.nikon.ca/app/answers/detail/a_id/15900/~/will-nikon-software-be-compatible-with-mac-os-10.6-%28snow-leopard%29%3F
3.  Some users have been able to use an older version of ViewNX which is the Nikon software in Snow Leopard successfully.  Click here to download that software.  http://www.opendrivers.com/driver/249985/nikon-viewnx-1.0.4-full-macintosh-english-free-download.html If that version doesn’t work, continue increasing the version number upwards until you find one that works.  The poster didn’t mention which version he used that worked.