Pokemon pearl!? PLZ HELP?!?

so ihv got poke radar and i want to catch a ditto. so i know where to catch it (its in the gr patch b4 cave city) and the problem is, i dont kno how to use ir! whenever i go to the gr and i then i go the bag and use pokeradar and it says 'rowan's words echoed theres a time and place 4 everything but not now' so i how do i use it? plssss answer.