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)"
But you would add the lex.close after the lookup_synsets() line, correct?
My code is failing on the line containing the lookup!