How to connect my iMac to the UPS - the plug won't fit

Your Mac can't read the UPS just through the AC connection. That would require a USB or other connection between the UPS and the Mac. Without knowing the specific model of UPS you have I can't offer suggestions, though APC has been rather notorious over the years for their weak support of Macs.

Regards.

+1 on Varjak's response. The UPS must be connected to the Mac via USB, without that type of connection the computer has no way of knowing the UPS exists. Once you have made that connection then you can "read" the UPS. If the model of UPS does not have a UPS connection, my recommendation would be to return it and buy one like an APC model that does.

My apologies, I assumed that you knew that in advance.

Visit Amazon and buy a plug adapter!

Excellent, glad you are on your way!