Hi,
In order to debug a BAdI implementation, you need only to put an external breakpoint inside the main method of the implementing class from se18. This method is useful when you know BADI.
Or
If you dont know BADI then you can do in one way by going to the TCode SE24 and enter CL_EXITHANDLER as object type and then go to 'Methods' tab , Double click the method 'Get Instance' to display it source code and Set a breakpoint on 'CALL METHOD cl_exithandler=>get_class_name_by_interface'.
now just run RSA3 it will be in debugging mode.
Or
As suggested by hasan , you can goto debugging mode and after reaching FM "CRM_ORDER_READ", then press SHIFT+F9 THEN GIVE CLASS "cl_exithandler" AND METHOD AS "get_class_name_by_interface". NOW YOU WILL GET INTO DEBUGGING OF BADI.
sRIKANTH.