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:

1. Select notes
2. In the Articulations/Ornaments palette, double-click on the tenuto symbol