A component needs a field for each of its owned classes, as explained in Declaring the class fields. In this case, the calendar needs a field of type TFieldDataLink for its data link. 

Declare a field for the data link in the calendar:

type
  TDBCalendar = class(TSampleCalendar)
  private
    FDataLink: TFieldDataLink;
  .
  .
  .
  end;

class PACKAGE TDBCalendar : public TSampleCalendar
{
private:
    TFieldDataLink *FDataLink;
    .
    .
    .
};

#include <DB.hpp>
#include <DBTables.hpp>

Before you can compile the application, you need to add DB and DBCtrls to the unit's uses clause.