Stacatto Help

It's exactly the same as anything else in the palettes—you select what you want to apply it to and then double-click the palette icon (or alternatively drag it out of the palette directly onto the score, but that's finicky even if you zoom in). In this case, you would select the note or notes you want to mark as staccato and double-click the staccato dot in the Articulations & Ornaments palette.

There is also a keyboard shortcut once you have the notes selected: [Shift]+[S].