I'm somewhat surprised by how sloppily prepared the MDL extension seems to be. Some of the first issues that have immediately jumped out at me:
The drumset definitions from the extension's instruments XML file don't use the sounds from the extension's SFZs. (And why is this special instruments XML hidden? The default instruments.xml is shown in Preferences / Score.) The symbols in the palette have no positioning rules (see attached screenshot). This whole workspace doesn't seem to have been thought about—the Fingering palette is removed, but the Fretboard Diagrams palette included, and so on.
All this makes me even more concerned about the templates and style sheets, which won't even have a beta period.