Hello,
I searched the manual and asked google. Maybe I'm stupid, but could someone tell me how to add credits in this game? There are only the "game start" and "launch" buttons, but is there a way to add more credits?

I don't think there's a key, but FWIW, here's the stuff you may know already:
If you get any kind of Best Performance (there are a few varieties), or your score exceeds the current replay threshold, you'll win a credit (as indicated by the distinctive loud 'clack' noise).
You also have a 10% chance of getting one after each game, so even if it takes you a while to achieve replay-worthy scores, you'll accumulate 'em over time randomly.
You can use the operator menu to modify the replay threshold score.

I believe you can also modify the match award percentage via the operator's menu - mine is set at 20% or so, I think. Also, multiplayer games have more scores to match against, and so appear to get match awards more often.