The "Fields To Aggregate" will be detail-record fields. As long as you are creating your Rollup field on the Master object you will see the available fields.
I was able to create a Roll-up field using the edit option of the sObjects. It still shows blank on 'Field to Aggegate' when I try to edit it with the Schema Builder. I don't really know why its not showing up there (Schema Builder).
The "Fields To Aggregate" will be detail-record fields. As long as you are creating your Rollup field on the Master object you will see the available fields.
I was able to create a Roll-up field using the edit option of the sObjects. It still shows blank on 'Field to Aggegate' when I try to edit it with the Schema Builder. I don't really know why its not showing up there (Schema Builder).