I found this question through google: mathtools.sty not found in kile and the answer in it solved my similar problem. The question is closed as too localized.
I believe that the question is not too localized, since the problem was not unique to the OP of the question.
So should the question be reopened? I understand that it will probably not benefit from new answers, since it is already well answered, but if it remains closed, it can potentially be deleted, which would be detrimental for other people with the same problem.