Need to know how to set up WiFi for FireHD 7 on hardwired iMAC desktop
If you have working internet over ethernet into your iMac then all you need to do is to switch on Internet Sharing from the System Settings app and presumably your Fire HD will be able to connect to that (I don't have a Fire HD so cannot explain that end of it).

Check out http://support.apple.com/kb/PH13855