I've just started using the product and would like to add custom icons to some items. Your guide indicates this is possible, but the link entitled "add a custom icon" is broken.
When you are in edit mode for a record for which you want a custom icon, just drag the icon onto the icon field and drop it there.
Thanks for bringing this to our attention! Our document guru is still working on polishing up a few tutorials - looks like you found one that isn't quite live yet. I've mentioned to him that you'd love to learn more.
In the meantime, I hope that @hawkmoth's description helps.