You can display it any where playing around selection screen statements like the comment statement and skip statement in selection screen.
http://help.sap.com/saphelp_nw70/helpdata/en/9f/dba7a135c111d1829f0000e829fbfe/content.htm
If you dont want to display it below the parameter as given above use the skip statement.
SELECTION-SCREEN beginofBLOCK bl1 WITHFRAMETITLEtext-001..
PARAMETERS: bukrs type bukrs .
PARAMETERS: matnr type matnr.
SELECTION-SCREENendofBLOCK bl1.
SELECTION-SCREENSKIP4.
SELECTION-SCREENSKIP2.
SELECTION-SCREENbeginofBLOCK bl2 WITHFRAMETITLEtext-002.
SELECTION-SCREENCOMMENT /10(30) comm1.
SELECTION-SCREENCOMMENT /10(30) comm2.
SELECTION-SCREENendofBLOCK bl2.
AT SELECTION-SCREEN.
AT SELECTION-SCREENOUTPUT.
ifnot bukrs isINITIAL.
if bukrs ne'1000'and bukrs ne'2000' .
comm1 = 'Enter valid company code'.
else.
clear comm1.
endif.
endif.
if matnr CA'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.
comm2 = 'Enter valid material no'.
else.
clear comm2.
endif.