I assume you created the MetaSoundSource variable and set you metasound asset as the default value. But you need to add an “audio component” to your blueprint or run the a function like “Spawn Sound At Location” to create an instance of an audio component that is using your meta sound asset.
When you drag off from the audio component reference variable the search should give you proper nodes. Also be sure to have the “context sensitive” checkbox activated next to the search bar. 
before you fall into that trap: you can’t call the default OnPlay trigger in the meta sound, use the “play” function for the audio component. Only input triggers you created can be used with the “Execute Trigger Parameter”.
Just for clarification:
Look into Blueprint Interfaces. The envelopes indicate that the node is an interface message function. You can throw any object in the Target input and it will try to find the needed interface class on that object. If your target object doesn’t have the interface added, nothing will happen.
The Yellow input demands an object that is guaranteed to have the right interface class added. So you can’t even put an object reference in there that doesn’t have the interface.
My understanding is that a message function just tries to cast the object to the interface class and calls the function. You can do the same with a “cast to” node and executing the function with the yellow input node.