Getting datamapper up and running on rails3 is really easy. It won't take longer than 2 minutes, really :) Here's how:
gem install i18n tzinfo builder memcache-client rack rack-test rack-mount erubis mail text-format thor bundler gem install rails --pre
To get this sample app installed you then have to issue the following commands. Note that it's not necessary to tell bundle install to install to the vendor directory. It's purely a preference of mine as it keeps all things related to the application, in one place.
gem install bundler git clone git://github.com/snusnu/datamapper_on_rails3.git cd datamapper_on_rails3 bundle install rake db:setup rails server
Point your browser to localhost:3000/people and you should now see rails3 serving an empty list of people. Feel free to add some.