Once a dim string is created you can drag it to another location with the selector (Select the selector tool first by typing the letter A.), and change its line and text formatting (text size and color and line weight and color) with the formatting toolbar buttons. Once it's placed it appears on top of a horizontal line, and, for a vertical line, to the left or right, depending on whether you pick the top or bottom point first.
Anne Marie, Autodesk Technical Publications