When you purchase DayPilot Pro for ASP.NET WebForm you receive a download link to a full version of the package. You need to use DayPilot.dll that was included in the full package download.
Sometimes the old version of the dll gets cached somewhere. You can try to remove the old DayPilot.dll first, try to run the project (it will complain about missing DayPilot.dll). Then copy the new version back and run the project again.