How to add items from the DMG to character sheet?

So I run a D&D game, and I wanted to give "Sending Stones" to a character... but when I search for it in the Compendium in order to drag/drop it onto their character sheet... it's not found. Oddly, other magic items are, like vorpal swords and Bag of Holding. Doesn't seem to be rhyme or reason why some items are available and some are not. Is there a workaround I can use to be able to add magic items that aren't in the Compendium to character sheets?

Sending Stones are not in the SRD (free info). They are in the Essentials Kit (available for purchase). The DMG is not yet available on Roll20, but should be this year.