If you press the enter key when the plan has the focus, an editable tooltip will be displayed to let you enter with the keyboard the length and the angle of the current dimension (if you didn't start any dimension yet, the tooltip will display the coordinates of the start point of the new dimension), then its offset i.e. the length of its extension line. Use the up arrow, down arrow or tab keys to edit an other value and press the enter key to confirm the values you entered. If you want to use again the mouse to draw a dimension, simply move it over the plan.