A user has reported that, when using the Dropbox Experimental version 0.8.x, in BookMacster, when clicking the Dropbox button in Settings ▸ General, BookMacster incorrectly reports that Dropbox is not available.

This has been fixed for the next version.  For now, the workaround is: Instead of clicking that button, just go directly to the Agents tab and (if you're using Simple view), just check the bottom box.  In the sheet which asks what type of cloud syncing you use, click "Other".  Then navigate to and choose your Dropbox folder.  Of course, if  you choose a subfolder, make sure that your Dropbox Experimental Selective Sync is turned on for this subfolder.

So, in other words, you're acting as though you're using a different cloud-syncing service, not Dropbox.  It should work fine.