Topic: Migrating from 1.5.4 to 1.6.2 or later
Due to changes at Mac App Store since June, 2012 only sandboxed applications are allowed.
Sandboxing is a security measure and means that application can access only it's own files inside dedicated location ('Sandbox'), so it cannot hurt anything other on the system.
Since our database was in the ~/Library/ToDoLists, or, even more, user was able to move it to other location (to achieve Dropbox synchronization, for instance), new sandboxed version cannot access it and move to the sandbox automatically.
It needs some attention from user - and it shows dialog, asking to open database file (when file is opened by user via Open Dialog, it can be accessed).
You should do this only once.
If you cannot open the path which was shown in dialog (for instance, when it is in Library folder, which is hidden on Mac OS 10.7 and later), please use the Finder - open it, use Go->Go to Folder.. , type path there, navigate to folder, and drag ToDoListsDatabase.tdl file to your Open dialog.
Or, the easier way: open Finder, in Go menu select 'Go to folder..', and enter ~/Library/ToDoLists (exactly as it is written here, with tilde symbol!). In that folder you should see some .tdl files - main ToDoListsDatabase.tdl and some backups.
Double click on ToDoListsDatabase.tdl to run To-do Lists application and load this database file to the main location.
Also you should re-configure Dropbox syncing via Sync page, it should work much better.