That looks like it's a Custom Report Type (CRT) you'll need to edit the CRT Layout and manually add the field to the CRT, then it will be available in the Report.
(you owe me a beer)Why are some expected fields not visible when creating a Custom Report Type?
Knowledge Article Number: 000005196Description
Why are some expected fields not visible when creating a Custom Report Type?
Salesforce offers Custom Report Types.
Available in: Professional, Enterprise, Unlimited, and Developer Editions
Resolution
Unlike standard report types, Custom Report Types any new fields you add to your object aren't automatically added. You'll need to add them manually.
Setup: Create: Report Types: Click on the Report Type's Label: Click "Edit Layout" within the "Fields Available for Reports" box on the right, you'll see the "Field Layout Properties" section. You can drag and drop desired fields to this layout.
If a field you wish to include is not visible under one of the selected objects, you may be able to add it via lookup. To do this
- Select the respective object from the "View" drop down field on the right side of the page.
- Click on the "Add fields related via lookup »" link directly below the "View" drop down field.
- Select the appropriate link and subsequent fields.
- Repeat this process as needed per object.
Note: To create or update Custom Report Types, you must have the "Manage Custom Report Types" User Permission.
