Changing from windows to Mac., I was advised by Adbobe to download CS 5 from their site. I did so and installed it.
After deactivate CS5 on my windows computer, I want to activate CS5 on my iMac. The number I used (from the box I bought 3 years ago for over € 950) did not work.
What should I do to get the right activation code?
You probably need to get back in contact with whoever advised you to download the Mac version and have them provide you with a working serial number. Normally platform swaps are not supported for anything but the most recent versions, currently CS6.