Getting the wordnet gem, version 0.0.5, working is a little tricky at the moment. It's being rewritten but isn't released yet and the dbd gem is in a similar state.
These steps got it working for me on OS X Lion and MRI 1.8.7. On 1.9.2 convertdb.rb segfaults.
Download WordNet from http://wordnet.princeton.edu/wordnet/download/current-version/