To delete files from your library you have to create an FTP user, you can do this through the 'My Account' section, option 'Users' by clicking the button 'New FTP User'.
To make use of this FTP user you can download an FTP tool such as for example FileZilla (freeware).
You can download it at:
Below you'll find a screenshot of this software.
In the upper bar you fill in your credentials you'll find in the Addemar FTP user pop-up screen. (Address, User and Password).
Next you can click on the Quickconnect button. After establishing the connection the right window will list all files available within your Addemar account to administer. The left window will show your local computer's file structure. Now you can delete files by clicking the file you want to delete and push 'Del'.
You can also upload by dragging files from the left to the right window or download files by dragging files from the right to the left.