If I understand your question correctly, here is how you create your multiple folders.

Set the value of INSTALLDIR to \MyApp\
Right click on the [INSTALLDIR] and select the 'Add' menu option. This will allow you to create your subfolders.

Once you have your folders and subfolders, do the normal drag and drop to show the installer where to place your files.

If you don't want to use the DATABASEDIR, that's fine. Just don't give it a value (Found in the General Information Section).