Hi Jonathan,
Check the references in your project. Make sure you are loading all of the correct ones for XI. ReportSource is definitely a method of the viewer.
I had a look at a project and these ar ethe references I included:
CrystalDecisions.CrystalReports.Engine
CrystalDecisions.CrystalReports.ReportSource
CrystalDecisions.CrystalReports.Shared
Thanks,
Brian