When you want to upload files to the forum you either need to create a new Topic (you will see the Attachment options at the bottom), or you need to press Post Reply in a topic (not quick reply, because it does not have attachment options).
When you go to a topic (like: https://vansteinengroentjes.nl/DD35/viewtopic.php?id=2 ) you can see the files to download at the end of the post, which a line looking like:
all-classes.dd35 6.24 mb, 21750 downloads since 2014-05-07
Clicking those links will let you download the file. In the creator studio you can download a file for each of the categories (spell, monster etc) or for all at once. There is no option yet to download only one item if you have multiple, this is something on my todo list.
I hope this helps you, if not please let me know where it goes wrong.
Bas