Hi
EDocman uses Joomla 2.5 core ACL feature, so it works different than normal upload extension. Basically, you will need to access to a category, next to category title, you will see an upload link . Click on this upload link will allow you to upload document for THAT CATEGORY, you cannot change the selected Category from Category dropdown.
If you change your mind and what to upload documents for a different category, you will need to access to the new category and press upload link (next to that new category title) .
Hope my explanation is clear to you. Please let me know if you need a more detailed explanation .
Regards,
Tuan