If you want the details in the subreport to be based on the record in the main report, here's what you do:
1. Right-click on the subreport and select "Change Subreport Links".
2. In the top-left, select the field from the main report that identifies which subreport data you want to display. Move the field to the top-right.
3. In the bottom-right select the field in the subreport that should have a value equal to that of the field from the main report.
This will create a parameter in the subreport that will automatically be filled with a value from the main report. When you run the whole report, you will not see any prompts for the subreport because the main report will set the value for the parameter. However, if you run just the subreport alone to test it, you will have to set the value of the parameter.
-Dell