Haha, awesome! My whole school is playing nDOOM and pokémon on the Nspire

Did you make sure not to transfer the Ndless files to the directory MyLib?

I didn't. I made 3 folders. ndless 2.1, ndless 2.0 and ndless 1.7. In those folders I pasted the install and resources files. (ndless_installer_os-1.7.tns in ndless 1.7, ndless_installer_os-2.0.1.tns in ndless 2.0 and ndless_installer_os-2.1.0.tns in 2.1)

Did you try it just one time each?

Nope, I tried it a couple of times. Sometimes there is a harddisk with a triangle and a exclamation mark in it and sometimes the calculator just freezes and is rebooting.

if it looks like the attached pic then it has nothing to do with ndless.
else could you give us a more precise instruction?

And the icon you send me isn't the same as the one on my screen. When I try to make a screenshot in TI-Nspire computer Link it doesn't work because the program doesn't recognize my calculator.

Well, I will give you a precise instruction 
- I first deleted the os and all the files (home + p + enter + on, option 4, pressed 1)
- Then I connected my calc with the computer for OS 2.0.1.60 (TI-nspire Link -> extra -> send OS)
- When OS installed, I made the three folders and putted the files in them.
- And then it happend.
- The same happend using 2.1.0.631 by the way...

If you want to know more, just ask me