First download the Unity Export Plugin from the link above.
Extract the Zip file into the desired unity project.
The Modest3dExporter.dll should be placed under an Editor/ directory as it is in the zip file
Right click on your saved scene > Modest Tree Export
Select the desired export platform.
Select the desired output directory. This would typically be the project directory you are using with Modest3D Editor/Xplorer.
Make sure all objects are set to only have lightmap static and reflection probe static.
All lights need to be set to bake and bake at scene.
A reflection prob should be included in the root of the object
keep the scene lights separate form the prefab containing the models
Delete the lights after you have finished baking
If you have upgraded your project from an older version of unity then 2018 LTS then you will need to check that the Scripting Runtime Version in the PorjectSettings > Player is set to .NET 4.x Equivalent.