How to add a calculated field to a report

In the report designer, select the Field List for collectionDataSource1

Right click any field in the field list and select
Add Calculated Field

Right click any field in collection data source and select Edit Calculated Fields

Edit the properties for your calculated field.
Make sure to set the Data Source to be collectionDataSource1

Click Into the Expression Editor to set the expression for the calculated field09Capture

Now you should be able to use your calculated field as a normal field on your report