It doesn't work with Mac OS X High Sierra.

If you updated your Mac OS X to High Sierra, you might have problems with opening the plugin in GarageBand. This doesn't affect only the MT Power Drum Kit, but also any other 3rd party plugin. The error message says:

"The Audio Unit XXX could not be loaded for your project, and may need to be updated. Contact the manufacturer for an updated version or further assistance."

Until Apple comes up with an update to fix this problem, you can try a workaround which is described here on this website.

A second possible solution:
If you can get your hands on a Logic version, you will be able to open your GarageBand projects there and all the plugins still work with the new High Sierra update. You should try the workaround from the link above at first though.

Update:
Apple has released an OSX Update in the meantime resolving this issue. Please look into that if you haven't already.