I just played with this and discovered the following.
Firstly, in a custom field text-box you put the name of your custom profile field. A custom profile field is a field that has been set up by admin to track something that it not normally in the profile in Moodle.
For example, I have a District field because I deal with multiple school districts. If I just put "District" (name of field) in the setup field, then when I do a grade export, I will get an additional column showing the District that my students attend. I have seen a couple of people saying that it is not populating but mine is. I would think that maybe the field name is not entered correctly.