Hey Tesztorum! Im guessing you have figured out by now that the model view file needs to be hosted on a web service (Preferably Dropbox) with the file being DOWNLOADABLE and PUBLIC; that should fix the problem... If anyone needs further help I've written an article which you can checkout down below on ways to create, host…
For certain browsers like Chrome, you need to run a local server to load local Viewer content, other browsers like Firefox can load it without running a local server. More information on that here: https://www.marmoset.co/support/#local There's also the integration guide here: https://www.marmoset.co/viewer/integration/