While we are waiting for approval from the QMK team to get our file integrated into VIA, you will have to manually set the layout definition in order to have VIA work properly for Monarch at the moment. It is very quick and easy though so just follow along these steps and please contact us if you have any questions. Keep in mind, that your keyboard will work just fine out of the box if you don't want to add any customizations to the PCB. It'll operate with the standard layout that we have pre-programmed. If you want to customize layers, keys, and macros, then you need VIA for that. VIA can be downloaded from this location.
If you open VIA with Monarch plugged in and see the "Searching for devices..." screen, look at the top toolbar and see if you have the "Design" option. If you do, skip to step 3. If you don't have the "Design" option, click on the "Settings" option and go to step 2.
On the Settings tab, turn on the "Show Design Tab" option. At that point in time, you should now see the Design option at the top of the window.
On the Design tab, we now want to click the "Load" button and select the monarch.json file we downloaded earlier. After doing so, you should now see the Monarch layout on the Design tab. At this point, everything is loaded and you can now click on the "Configure" option at the top toolbar.
From here you are able to customize your Monarch to your liking.
Have fun and enjoy!