The install from the repos is an old build, so you'll want to update that first, if you have not already.
You need to add the minetest ppa to your software sources
The ppa isas follows: deb http://ppa.launchpad.net/minetestdevs/stable/ubuntu
After you add that, refresh your software manager and there will be an update "minetest 0.4.13"
Once you have done that, select show hidden files in your file browser and look for .minetest
open that folder and create a new folder and name it games, if it is not already there, although looks as though it is
unpack the tutorial file into that folder and when you start minetest again, you should see the blue graduation cap button in the lower left corner..., click it n go
I think the problem is that the subgame is not compatible with the current build in any Linux distro repos and I wish they would all update them. When I installed minetest from my repo it was version 4.9