how do i transfer photos from my sd card to my imac?

Is the memory card in the camera or in a card reader for the Mac?

What camera model is it and what format photos are you shooting?

Does the card show up on the Desktop? If not make sure all checkboxes are checked in the Finders General preference pane:

Then it should show up on the Desktop and you can drag the folder containing the image files to the Desktop. Then import from there into your library.

If you launch Photos with the card showing on the Desktop it should be recognized by Photos in the Import view under Devices in the sidebar:

what happens when my sd card does not show up on photos ,

Is it from a digital camera with the DCIM format?

https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_system