Is it just the arm that's come off or has the complete bracket the arm clips into has come off?
Can you post the model number as well please.
arm that the water looks like it comes out of and a little column thing that looks like it goes in the middle of the arm.
Bosh classi x x SRS3A02CB/08 slimline dishwasher
Ok if its just the arm part then in the middle of the arm is a hole.
Line this up the bracket under the bottom basket in the middle and then push that column thing you have up the arm and then twist it and this locks it into place.
Have a look at this film and this should help you more.
If you need anymore info just get back to me.