I was prescribed adderall and I'm now having trouble sleeping. Is it safe to take melatonin to help me sleep?

Ask your doctor.

Besides, your doctor needs to know that you can't sleep. You may need a reduction in the amount of Adderall, which is a stimulant, or even a different drug. Don't try to counteract one side effect with another drug, even if it is something 'harmless' like melatonin. Nothing you put in your body that isn't found in it's natural state and concentration is harmless.