1. Create a parameter field in the main report.
  2. From the main report, right click on the subreport, and select Change Subreport Links.
  3. In the subreport linking window, select the parameter field from the window labeled "Available Fields", and click on the ">" button to move it to the window on the right labeled "Fields to link to"
  4. Below, make sure you have a check next to Select data in subreport based on field.
  5. Below the checkbox, if it is not already there, select the field from the subreport, to which you would like the parameter field linked.
  6. Click OK