I am assuming that you have already created at least one surface profile and one design profile. The picture below shows how I set it up to create the report on the right side.
In Reports Manager>Select Incremental Station Difference Report>Check the profiles you want compared in difference report>Station Interval>20.
If you only want the even 20 m. Intervals be sure to uncheck all check boxes on bottom of dialog except for "Regular Interval" will ned to be checked.