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/
$ ./configure
$ make
$ make install
This uses homebrew (http://mxcl.github.com/homebrew/). If you use a different package manager then you will have to adjust the --with-db-dir below.
$ brew install berkeley-db
$ git clone https://github.com/knu/ruby-bdb
$ cd ruby-bdb
$ git checkout 6aa8054e022fa38055fd441de98b65fd392b8dcb
$ ruby extconf.rb --with-db-dir=/usr/local/Cellar/berkeley-db/5.1.19
$ make
$ make install
$ gem install wordnet
$ gem which wordnet
$ cd your/wordnet/path
$ ruby convertdb.rb
$ mv ruby-wordnet/ `ruby -e "require 'rbconfig'; print Config::CONFIG['datadir']"`
$ irb
> require 'rubygems'
> require 'wordnet'
> lex = WordNet::Lexicon.new
> frog = lex.lookup_synsets('frog', WordNet::Noun)
> frog.first.hypernyms.first.overview
=> "amphibian [noun] -- (cold-blooded vertebrate typically living on land but breeding in water; aquatic larvae undergo metamorphosis into adult form)"
It could very well be that Berkeley DB isn't installed properly. Can you confirm that the Homebrew install is working?
Also check that the version is correct by
ls /usr/local/Cellar/berkeley-db/
.