What you need to do is send AC customer service an e-mail / letter asking for the compensation.

I usually send something along the lines of "flight 123 from abc to xyz was delayed on Jan. 1 due to a mechanical defect and I arrived at my final destination with a delay of x hours. Since the delay was within AC's control, I am asking for 600 Euros compensation in accordance with EU guideline 261/2004".

Keep it short and to the point.

Generally speaking most airlines refuse to pay any compensation claiming that mechanical issues are an extraordinary circumstance and that no compensation is due, so don't expect the airline to be overly cooperative, but sending a message to CS is the first step you need to take.