You can show, or hide the palette using the menu Display → Palette (or use the shortcut F9).
You can drag-and-drop palette symbols onto score elements.
Double clicking a palette symbol is equivalent to drag-and-dropping it to each of the selected elements on the score.
For example, you can quickly add tenuto marks to a group of notes:
- Select notes
- In the Articulations/Ornaments palette, double-click on the tenuto symbol