What you do to highlight a particular Marker depends on your existing code. I assume you want to be able to highlight an existing Marker.
Traverse your Marker information. Filter it based on the criteria you expect to use to 'locate' or highlight the Marker on your map. You can also hard code the information to highlight a Marker by it's 'name' if you already know which one you want to 'locate on the map'.
Change it's color or center the map on the Marker or Marker.ShowInfoBox etc. using a Button or ListPicker or ListView event handler. This will make it easy for a user to find the highlighted location.
The following two part tutorial contains a examples of several ways to highlight an existing Marker.