user exit QEVA0008

hi sap experts.

i have a problem in activing a user exit.

refer to SAP Note 33924 , i want to implement the 'Cancel usage decision' function via User Exit QEVA0008.

i search and find this :

activat an existing SAP user exit (SAP function EXIT_SAPMQEVA_008, Include ZXQEVU10, Include LXQEVF10). Activating this function causes a new button (Reset Usage Decision) to appear on the toolbar of the QA12 transaction

so , i copy this code to ZXQEVU10 and active it. but i dont have  button (Reset Usage Decision) again.

   ***INCLUDE LXQEVF10.

*-- Example: Reset the Usage decision
*-- Please add your own authority requirements here or
*-- use the authority check within the general status management
*-- by adding a user status profile

*-- Reset Status Usage Decision by using business transaction QM68.
CALL FUNCTION 'QAST_PROCESS_ACTIVITY'
     EXPORTING
*         I_DIALOG             = 'X'
          I_OBJNR              = I_QALS-OBJNR
          I_VORGANG            = 'QM68'
     EXCEPTIONS
          NOT_ALLOWED          = 1
          ACTIVITY_NOT_ALLOWED = 2
          OTHERS               = 3.
E_EXIT_ACTIVE = 'X'.

when i see in SMOD, enhancement " QEVA0008 " , select component radio button ,DBL click in +FC1 , in function lists,customer functions , status of function code  +FC1 is"  inactive reverse", what can i do to active it?

thanks for your helps.