Nintex Workflow 2010 - Update Item - Infopath 2010

Hi, I am new to Nintex and trying to use the Update Item action.

I have created an InfoPath 2010 form with a status field I want to update. The field appears in the SharePoint Library column but I can't select it in "Field to Update". The only items are Content Type, Name and Title.

 Can anyone help.

Thanks
Jeff