The left box Andreas was referring to is the left panel in write mode, the one where you pick the rhythmic values of the musical objects you want to input. You can select a value (for instance a quarter note), enter note input mode (by double clicking where you want to input your dynamic in the score — it will show the caret), then press shift-D to input a dynamic there, input whatever dynamic you wish in the popover (any hairpin < or >, or crescendo, dim., whatever you need), press enter, and then press the space bar. This last step will make the caret move and write the dynamic according to the rhythmic value chosen at the beginning. A nice trick is to select a short value (16th note) and press the space bar as many times as you feel in order to get the dynamic’s length that you wish.
Hope it helps !