How do I turn on my imac using my wirless keyboard?

Arch,

Unfortunately, that is not an option even with a wired keyboard.

The only power switch is on the back of the computer, use that. However even better is put the computer to sleep when it's not in use. Then when you are ready to use it tap on the keyboard spacebar and wake it up. You can set that up in System Preference - Energy Saver.