In io_object_mu-master (not io_kspblender-master) theres a file called import_craft.py (same name as in kspblender)
Line 37 says something like: gamedata = "/home/bill/ksp/KSP_linux/GameData"
change "/home/bill/ksp/KSP_linux/GameData" to where your game data is (you can use Steam, properties, browse local files to help)
be aware that for windows / is an escape character and thus an extra / must be added, mine looks like
gamedata = "C:\\Program Files (x86)\\Steam\\steamapps\\common\\Kerbal Space Program\\GameData"
The error comes from the project kspblender is built upon due to hard-coding a position for GameData (importing a .mu wouldn't work either beforehand) I had the same error and this fixed it for me so hope it helps.