virtualbox.org

I cannot seem to find a way to attach an ISO image to a virtual machine, in previous versions there was an option to configure an ISO file as a CD drive.

Has the option been left out of the latest release or am I being naive?

I am using V.5.0.12

The option is still there. Open the VM Settings, Storage, select the CD icon from the left side, click on the CD icon on the right side, select your ISO.

I expect the OP is trying to add the ISO as a hard drive. You need to include a CD drive in the VM recipe, then you can mount ISO images.