Hi
1. The upgrade process you did is incorrect. You will need to import the database tables from Joomla 1.5 website into Joomla 2.5 website first, then install latest version of the extension to your Joomla 2.5 site. For now, please install latest version to your Joomla 2.5 site again .
2. For the format of the date, you will just need to go to Documents Seller -> Configuration, find the config option Date Format, change it (remove % character from the config option) and everything will work as expected .
Tuan