How to add custom field to an non-header block through AET?

I am trying to add a custom field to an existing block through AET. All AET demos seem to focus on adding fields in header page od documents which are done in a jiffy. When we try the same method to add fields in say, 'transaction history' block of quotation, it does not work. After making sure its a configurable area, I add a field and generate it through AET. But when we come back to available fields, the newly created field does not show up. What am I missing?