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 or Ubuntu 10.04 and MRI 1.8.7.
Download WordNet from http://wordnet.princeton.edu/wordnet/download/current-version/
$ ./configure
$ make
$ make install
for ubuntu, first install tcl and tk libraries
$ sudo apt-get install tcl8.4 tcl8.4-dev tk8.4 tk8.4-dev
and compile with
$ ./configure --with-tclconfig=/usr/lib/tcl8.4 --with-tkconfig=/usr/lib/tk8.4
$ 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
for Ubuntu
$ wget http://download.oracle.com/berkeley-db/db-5.1.29.tar.gz
$ tar xvzf db-5.1.29.tar.gz
$ cd db-5.1.29/build_unix
$ ../dist/configure
$ make
$make install
the location may change, but for MacOs is: /usr/local/Cellar/berkeley-db/5.1.19 and Ubuntu: /usr/local/BerkeleyDB.5.1/
$ 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)"
Don't forget to close the connection for avoid Segmentation Fault
> lex.close