In order to load the .mbtile file within the application, we just have to click on the + button from the table of contents and then select the option Add → Raster Layer → Load MBTiles / Rasterlite file.
We select the file to load and define the characteristics of the new layer.
We have a post on our website that details the creation of this type of files compatible with CartoDruid and their loading in the application:
https://www.cartodruid.es/creacion-de-mbtiles-desde-repositorios-online