You will need to mount the image using your password. Then you can double-click on the database file to open it under DT Pro (if you have another database open, it will be closed and the new database will be opened). If you plan to add new content, make certain the volume is large enough to hold the additional content, with some additional room for file expansion, including the internal backups.
Always close the database (or switch to another database) when finished working with it, to make certain that anything still in memory has been saved. Then unmount the disk image.
As always, I would suggest an external backup. In this case, an archive copy of the encrypted disk image should be made, perhaps to an external drive, CD or DVD.