Try this if you still want: Go with your file manager to:
Inspiron-600m:/home/celeron55-minetest-25cf375/games
Under tools (upper line of your file manager, you have the option to open the current folder in the terminal. So do it.
then try again this line:
WITH https after wget and before
/
wget
/github.com/celeron55/minetest_game/tarball/master -O master.tar.gz
I took https out for this post, because the forum keeps making a link of it. You must in your terminal put it between wget and
/ but attached to
/
Sorry for this way of writing but I'd hate to make typo's