Ubercart 5.x is for Drupal 5.x. Ubercart relies on modules in Drupal6 that have not been ported to it. If you need Ubercart or an e commerce module within the next few months I would go with Drupal 5.7 as all the modules you will need are set up for it.
Look at this thread http://www.ubercart.org/forum/development/1584/drupal_6