How can i get a dynamic mic to work on iMac?

Well yes, if it is a USB microphone, the assumption is to plug it in. Note, Apple provides an Audio Midi Setup utility in Applications -> Utilities that once plugged in expands on the settings in Apple menu -> System Preferences -> Sound -> Input

Some microphones require Mac OS X drivers. So it would be important to determine if yours is one. Google your make and model microphone with Mac OS X and the word drivers to see if anyone has needed it.

A usb input is not an audio input, so something would be needed to convert your analog mic signal to a digital one that the usb/mac can understand, usually something external (like a mic preamp to usb out)

Just plugging an analog mic into a digital input will normally result in nothing. So if that is what you get, it is working correctly, but not usefully.

What is the mic?