Ravenloft Monster Tokens

I purchased the Curse of Strahd campaign. On the maps some of the monster/npc tokens are invisible to the players and some are not. The ones that are visible to the players on the map are creatures that they can see upon entering a room or an area. The ones that are not are creatures that will surprise them or are hidden, etc. My question is, how do I, as the DM, activate or manipulate those tokens which are on 'invisible.' I'm sure I'm missing something obvious. 

Just keep them on the GM layer until the party encounters them.  Then move them to the token layer.  Surprise! If the monsters/npc's stay invisible for the encounter, there are a few things you can try. Curious about token visibility Anything better than using GM layer for invisible Pawns? How do GMs deal with invisibility on Roll20?

Thanks Vince. How do I move a token from GM layer to Token layer? I toggle between GM, Token, Map and the token remains invisible to players and somewhat translucent to me as the GM versus completely clear like the other tokens.

Tokens will be somewhat translucent when they are on the GM layer.  It's to help indicate that the players cannot see them.  You can right-click tokens (you can select multiple tokes as well and even group them if needed) Layer|token layer.  If you enable "  Use advanced keyboard shortcuts " from the sidebar Settings tab, you can use " l o " letters "l" and "o" to send objects to the token layer.

Hello, you have to be on the gm layer to select the tokens. Then just like Vince said :)

I'm on a Mac that's why I couldn't figure this out. Ha.  Easy. Thanks.