Sometimes my PowerPivot field list is grey and can't be selected. It can happen when you are busy in an existing PowerPivot sheet. It looks like a bug because a restart of Excel solved my situation.
- Save your Power Pivot Sheet
- Close all other Excel sheets
- Close Excel
- Check in task manager if Excel.exe is not running
- If Excel.exe is still running, kill this task
- Start Excel
- Open your Power Pivot sheet
- Click on a graph and you should be able to select the Field list.