Hello Karina,
I just checked and this issue happens because the classes structure is the old one. There are two ways out
1. You could activate the class and new Abap workbench will save the class in the new format and then you will able to proceed with the above implementation. But this will be a modification.
2. You could go the way of Implicit enhancements
http://wiki.scn.sap.com/wiki/display/ABAP/How+To+Do+Implicit+Enhancement
The above example is for function module but you could follow steps for Abap method. The only restriction here is you can add code at beginning or end of the method only.
You could choose the way forward.
Thanks and Regards,
Veera