You need to click the "New" button and type in the name of the attribute that you want to add. Then click "OK" and select the type, set the value, and you're done.
Attributes are a general mechanism to add things that aren't directly supported by Anim8or. There are many applications that read and write .an8 format files. Attributes allow those apps to export models to Anim8or to be edited and read them back in while retaining arbitrary other information that they may need.
I used them to extend materials for the initial ray tracing renderer. Yes, I know they aren't as convenient as built in buttons. But since the ray tracer is still getting new features attributes are a fast way to support them. Writing the dialog code is very time consuming so this allows me to get the ray tracing preview out much sooner and to not waste time rewriting the material dialogs each time the ray tracer changes.
