Dear Senthil,
Then you can use mmpur_message_forced and also include the
include mm_messages_mac in your check method.
like...
METHOD if_ex_me_process_po_cust~check.
include mm_messages_mac.
BREAK SENTHIL.
* S PO Creator should not release the PO - Senthil 05/14/2014
DATA : lt_hdr TYPE mepoheader.
IF sy-tcode = 'ME29N'.
CALL METHOD im_header->get_data
RECEIVING
re_data = lt_hdr.
IF lt_hdr-ernam = sy-uname.
mmpur_message_forced 'E' 'ME' '303' text-002 'User is not authorized to Release Purchase Order' '' ''.
ENDIF.
ENDIF.
* PO Creator should not release the PO - Senthil 05/14/2014
ENDMETHOD
Many Thanks/ Himanshu Gupta