Adding a dropdown list or radio button list to the checkout form.
The only options available are text, text area, checkbox, date/time, number, address and order adjustment.
How do I add a dropdown or radio button list to the checkout form?
There are quite a few (free) attribute add-ons on the marketplace that could be used as an example. These would give you a lead on how to install an attribute.
Also you could look at /concrete/attributes, to see how the built-in attributes are made.
check it here:
I have tried several different searches and urls to find this, including "http://www.concrete5.org/marketplace/addons//dashboard/system/attributes/types" but I'm not having any luck. I am new to Concrete 5 and don't know much about it.
Here you can enable the select attribute for pages or users.
After this you can create an attribute of this type.