Just thought to add the little description on how to install and use this add on.
First make sure which version of LE you are using, there are 3 versions at the download link. choosing the wrong version results in not working though
Start librelec rr (or your LE version)
Click on addons in the left menu.
In the upper left corner click on the box icon.
Choose install from zipfile.
Browse to where you put the makemkv zip file (this could be usb drive, harddisk or network drive)
Wait for it to install.
Start the add on at least once to update/install the key! (Click on Addons in the left menu, and click on makemkv, popup will inform you)
Reboot
Make sure in LE settings that for Blu-Ray the simple menu is selected, blu-ray menus often don't work. It is under player settings/disc
Insert a Blu-Ray disc and wait until the disc menu appears, but don't use this disc menu, it doesn't work!
Click on video/files and select the line with your blu-ray disc and click play if you have a remote, otherwise right click on that item and choose play. Do not use enter or ok on your remote, it won't work.
After this, if all goes well, there will be a list of found titles on the disc, sometimes only one but it can be multiple ones. if multiple choose the title with 800 in the title, or the longest title with the most chapters is usually a good starting point