Are you using a version configured as a MOBI dictionary? This is the case with
Dictionary, Webster's: Webster's Dictionary 1913, v1, 22 Nov 07 and
v2.1, but Version 2.0 is not a MOBI lookup dictionary.
The DR1000S supports multiple dictionaries, so you could also try
Various Authors: GNU Collaborative International Dictionary of English, V1, 9 July 07 and
Wordnet Dictionary for Mobipocket ( Free).