mpk249on iMac .. Logic Pro doesn't see my midi keyboard mpk249.. please help

First if you are using a USB hub, try plugging in the keyboard directly into the iMac. First Restart Logic and once it is loaded, then plug in the keyboard. Hopefully it will bring up a notice that a new device was found.

If not, go to Logic Pro>Control Surfaces>Setup. when the new window opens, click on the New>install. See if you find your keyboard on the list. If not click on Scan and hopefully it will find your keyboard.