It is possible to link to a file in Dropbox, but the file must be in your Public folder (within your Dropbox account) and you need to use the Dropbox web interface to establish the link to the file because MindGenius requires an http or www prefix in order for links to work.
With the file moved to your Public folder:
- Right-click over the Dropbox icon in your system tray and select "Launch Dropbox Website" from the menu.
- Once the Dropbox website is open, navigate to your Public folder
- Right click over the file you want to link to and select "Copy Public Link"
- The URL will be copied to your clipboard, which you can then paste into the attachment field in MindGenius.