Right click the component type you are targeting in the Palette and choose Available Styles for ____
Then in the dialog box select the style you'd like to make available to this component type then save.
Select a component of the targeted type and you should now see the style available.
I've created a shared style (form), for an "Advice Block", but wish to make it available (as an "Add shared Style" option) to "Promo Block". Is there a way to configure this?