I have a subscription to elements.envato.com I have downloaded a web template package called "spectrum". But I can't open it with Muse however it can be opened with Dreamweaver
this are the subfolders
what am I'm missing?
What made you think that you can open these files with Muse? Muse is not html editor. Muse can only open the project file that has an extension .muse.
It looks like this.
You purchased a web template not a muse template, you need a Muse file to open it in Muse and based on the screenshot you purchased the wrong item.