Q: can i locate my stolen imac using the serial number if i had not activated find my mac ..please help THANK YOU

No, you have to enable "Find My Mac", before a Mac is lost.

But be sure to report the theft and the serial number to the authorities.

See this link:  Reporting a lost or stolen Apple product

And check Kappy's User Tip for the precautions to take, when a device has been stolen:

What To Do If Your iDevice or Computer Is Lost or Stolen