Make sure you set the display quantization to a triplet value. You can set the display quantization from the Display Bar, Display Track or Display Score dialogs. If your display quantization is not set to a triplet value, triplets won't be properly displayed. Similarly, if your display quantization is set to a value greater than the durations you are trying to display, these smaller durations won't be displayed properly.
Select the duration you want by clicking on it in the durations palette and then select triplet as the duration qualifier by clicking once on the duration qualifier (the button to the right of the durations palette in the main control area).
To put a triplet bracket over the figure, click on the Symbols Button, go to Adjust and select the bracket symbol. Draw it in over the figure and adjust it using the control points. Enter a number by using the + and - keys on your computer keyboard. When it looks the way you want it to, press the ENTER key. To cancel the operation press ESCAPE.