You're almost there!

Now, enter your key to say you're ready, and then go to another player and press both input buttons at the same time. (For example you have "*/- player 5 : " somewhere, press * and - at the same time.) The word "computer" should appear behind the player. If not, keep pressing them, you'll get there...
If you want even more computer players, then press the up arrow button to enable all players at the right side of the menu.
Other human players can join by picking any other slot in the menu. It's always fun to play this with your little (or big) brother or sister, but it gets really fun when 5 or more play.
Hope this clears a few things, I should re-construct the menu and make it easier to use...
Quote: "Which I think is a bug... xD (I managed to go through my own line then tried it again once I thought it worked and died... ) "
I don't think it's a bug, it's the spacings. You may notice that random spaces appear on the snake. They look like thin strings. It's possible to pass through those spacings. And what I think is happening is that you got really lucky and went through a space that is too small to see...
TheComet