I have a Nexus 7 v2 (2013) running Android Marshmallow 6.01 and have installed the RM app. To test I used a small tree (5 people no media) on my desktop and backed the tree up to DropBox using RM 7. I can look inside DropBox and see the file using both the desktop and the Nexus. When I start the RM app on the Nexus I see the instruction screen and the got to Files and get the 'On Device','Dropbox','Help' menu.
I select 'Dropbox' ( I had already given permission during the ALLOW) . All I see is the RootsMagic header at the top of the screen and the menu at the bottom (File,View,Lists,Tools,Info). The center of the screen is blank with a dark gray background. I can use the Dropbox app on the Nexus and see the tree in the Apps/RootsMagic folder.
Found my answer - was using the Backup function which the app does not use