immanuela - Until recently I had been away from any Construct 2 activity. I've also previously taken my Kongregate games down. So I'm assuming nothing new has happened since I last was involved with C2.
Concerning the highscore, from what I can remember, you should see the highscore tab besides your game within the first few days if not as soon as it's up. I understand they've changed the interface, but there should be a highscore tab there.
My suggestions are, to double check that the variable name inside the C2 file is the same as you call it within the Kongregate interface when you set up the game. Also, try to avoid capitalised letters, I began only using lowercase in the variable names for Kong due to previous experience. On top of this, if you can have your C2 file update the score to Kong regularly (after each level etc..) with the highscore type being that of the players best score. If you only send the score at the end of your game, and if players don't finish the game, then the highscore table entry will be bare.
If I can find the time, I'll try to experiment a bit more to see if anything has changed.
Hope that helps... also if you still have difficulty, if you can provide more info, that'll help.