Got the Milviz 310R yesterday and I'm having an issue when trying to add buttons to Stream Deck that reference the Lvars. I'm pretty new to this so probably I'm just missing something and any help would be appreciated.

I can add the standard K: events no problem on the Stream Deck and they work fine with the buttons. I tried to add both a Toggle or On/Off button for a simple on/off windshield de-ice switch for example using write event = L: and took the name from the Lvar list in AAO after I had the Lvars read from the sim. I copied the same format that worked ok for the K: event buttons filling in the send value and the A: info but it doesn't work. The on button just flashes momentarily and the switch doesn't move or make any sound.

Is there some intermediary step that I'm missing? I can add the button for windshield de-ice using the Lvar to my Honeycomb Bravo no problem. Just can't seem to get it to work on the Stream Deck.

Lvars, Hvars and BVars are not the same thing as (K:) Events. Contrary to K:Events "Vars" don't "trigger" anything, they are just variables. It is very rare that you can use them the way that you did (that would imply that the maker of the aircraft has implemented them in such a way that sending a specific value causes the aircraft logic to do "something".

It is much more common that you need to write an RPN script around the LVar, that mimics what the aircraft is expecting from you. There is a good chance that the LVar is only there to control the animation of the button/lever, but not the actual functionality - and that you have to use HVars and BVars to operate it.

The usual procedure is to use the MSFS Developer mode, specifically the behavior debug options to find out what the aircraft does when you click on a specific switch. My own method of choice is reading through the actual behavior code in the ModelBehavior definition file of the aircraft (provided that is even possible, some aircraft have encrypted files). 

Or wait until somebody familiar with all these things posts a StreamDeck profile on flightsim to. Or ask the developer of the plane what you are supposed to do if you want to use external controllers with his product.