Download of Tripal is currently performed via the 'git' utility. Navigate to where you install modules in your Drupal installation and run the following commands:
git clone http://git.drupal.org/project/tripal.git cd tripal git checkout 6.x-1.0
You can safely ignore any messages about being in a 'detached HEAD' state.
The Tripal theme is now downloaded separate from the Tripal modules, and it is required to properly run Tripal. To retrieve it, navigate to where you install themes in your Drupal installation and run the following commands:
git clone http://git.drupal.org/sandbox/spficklin/1342972.git tripal_theme cd tripal_theme git checkout 6.x-1.0
Also, you can download Tripal Extensions and install each as typical Drupal modules to extend the functionality of your Tripal site.
See the Tripal Tutorial for Installation Instructions.