Make sure your game is using one of the sheets that support D&D 5e: D&D 5e by Roll20 (Official and most compatible) Shaped Sheet (Less compatible, but more powerful and easier to program for, doesn't support Charactermancer) Community Sheet (Simpler, but doesn't support modern features, abandoned. I don't recommend.) Depending on your sheet choice, you may need to manually choose the D&D 5e Compendium under your game settings (so that it knows which Compendium to use--you wouldn't want the Monster Manual showing up in your Pathfinder game). Once you have verified these are set properly, load up your game. On the side bar, one of the tabs is your Compendium Tab (looks like a circle with an "i" in it). Click on that. You can either click on the Monsters category, or start typing the name of the monster in the search bar. Drag that monster by its name onto the VTT. It's there and ready to go! Depending on your sheet, all of the monster abilities are clickable macros: attacks, saving throws, etc.
