How to retrieve the values from a document list and edit in the Task

Hi,

I am using designer 7.1.1

In my project we have a document list and we have to potray all one value from all the documents as an editable field to the task assignee.
we are not able to select the particular field and when selecting the field it is giving “Failed to evaluate binding expression” error.

Please help.

Thanks
Gayathri