Here's some info I found at Michigan State U. Extension Service - Preserving Food Safely section:

http://www.ccas.msu.edu/msueimp/htdoc/mod01/master01.html

DRYING SEEDLESS GRAPES

Leave the grapes whole or cut in half for faster drying.
If drying whole, dip in boiling water for 30 seconds to
crack skins. No further pretreatment is necessary. Drying
takes 24 to 48 hours in a dehydrator. Grapes are adequately
dry when they have a raisin-like texture and no moisture in
the center.

Hope that helps!

Betsy