Thorn Whip is missing

How would i add that cantrip?

If something is not included with the compendium, just add a new spell on your sheet and manually edit the fields.  You could even drag/drop a similar spell to your sheet and edit the fields to whatever you need.

Just to explain why Thorn Whip isn't available in the compendium:  The   5th Edition SRD Compendium   can only contain information (such as race and class options, feats, spells, treasure, monsters, etc.) that is located within WoTC's   Systems Reference Document . If WoTC releases more SRD content under an Open Gaming License, you can count on that being added to the Compendium.