Hi Wolfgang,
The Function you are needing is CHAR, but Numbers seems to be fussy about CHAR(10). I'd swear that I've used it in the past with success, but when I try it now I'm getting the error: "The formula contains a number outside the valid range". There are simple workarounds though.
What you will do depends on how you plan to use the return character in your expression. If you are simply wanting to type multiple lines of text in a cell, go to the Table Inspector and de-select the Return Key Moves to Next Cell checkbox. Now your return key will insert a return character instead of moving the cell focus to the next cell. Uncheck the box to go back to normal operation. Another way to insert a return character is to use the Option-Return combination.
If you want to use the return character in a calculation, you can create a cell with only a return character in it and reference that cell in your expression. I've found that selecting a cell and typing 2 Option-Returns will create one return character in that cell.
Hope this gets you on track for now. Again, CHAR(10) should work, but seems to be broken.
Jerry