In order to make the properties available for segmenting or using in emails, you'll need to ensure there's at least one subscriber with those properties in your subscriber list (any list). You can do this via import, or submitting a subscription form with that data.
With Custom Variables, you can use any of your customers' and contacts' data on your emails. You can select which properties, from those you have available as Custom Properties, will be available to use as Variables.
In Custom Variables, you can add your custom "Property" and "Variable":
Segmentable Properties dropdown: (top)
Variables Dropdown: (bottom)
You should also see some variables available by default by us:
Got more questions? Reach out to us at email@example.com