There is something i don´t get in twilight wenn Bella writes the Letter to Edward how did she know that he would find it. I mean How could Edward find the letter when Alice had it and she didn´t know that it is for him? Or did she know?

Edward would read Alice's thoughts and see that she has Bella's letter and since Bella escaped just before he got to the airport (i mean his aeroplane landed) he would probably want anything that had some kind of track or clue where she went...
Bella knew that and that's why she wrote that apology letter..
it doesn't matter if Alice knew or not that the letter was for Edward, the fact is that Edward would want anything that's Bella's, and he would see her letter in Alice's thoughts..

she would probably see him find it and then tell him.. :)

well first off wat NAHLA said and plus Alice can see the future so she might of seen certain things like bella addressing it too edward or seeing edward open it instead of her mom