hi,
now that I have the full game, I decided to download tracks.
I put them into the "downloaded" folder (like in TMUF) but a can't browse through my tracks within the game =(
if I go to "single player" I only have all the colors for the officials tracks and access to ranking.

do I have to open the track in the editor and validate it ?

Downloaded tracks from mania exchange etc from other makers will now have passwords since the game as gone full retail

goto edit mode
load maps
then you will see the arrow next you my maps
click it
which will open DOWNLOADED
now play !

so i am guessing the answer to your ? is yes

Local Play
Single Map
Downloaded
click play far right bottom of the corner

No, you don't have to open them in the editor. You can play them just normal.
Just you won't find the tracks behind 'Single Player'. You have to choose 'Local Play'.

Ok, but, is it possible to make an official time on other tracks than nadeo tracks?

no, you can just try beating author time.

There's an option called "Challenge other players" after you've driven a replay (it appears while watching the replay). I have not tried it yet though, no idea what it does.

In TMU it was possible to make official times on other authors tracks.
Though I don't remember if it was actually called 'official' race. But you could submit times on other authors tracks and they would count for your ranking.

I haven't tried it in TM².

Yes it was called "official time" and was very funny.