Many users have asked, "How do I use Embrilliance Thumbnailer?" The truth is, you never need to actively 'use' it to get the benefit of easily previewing your designs!
Embrilliance Thumbnailer isn't a standalone program in which you open designs, it's more like an upgrade for the File Explorer in Windows or the Finder in Mac. Embrilliance Thumbnailer gives your operating system the power to show you a visual representation of your embroidery files in any file selection or file management window.
When you install Embrilliance Thumbnailer, the window you are presented is the Control Panel. In this window, you'll select all of the files you'd like to see as graphical representations (Thumbnails) instead of plain icons in your file windows. Most users will click 'Select All' and then click 'OK' to tell Thumbnailer to create their visual icons. After that, the Control Panel has done its job; you very likely will not need to open it again unless Thumbnailer is updated.
Once Thumbnailer is properly installed, it will start creating icons for your embroidery files as selected. When it is finished, you'll see something like the below image in your File Explorer or Finder, provided you have set your view to see thumbnails or icons as shown in the menu bar.
You can't 'open' a design in Embrilliance Thumbnailer, because you'll never need to 'open' a design to see what it is once it's installed. Never assign filetypes to open in Thumbnailer, because it doesn't have any capacity to open files.
If you'd like software to open designs, resize them, add text, or otherwise perform basic embroidery software functions, you will be best served by adding Embrilliance Essentials to your software collection.
For more detailed information on Thumbnailer, please check the Embrilliance Download Page; there you'll find complete manuals and installation guides for Mac and Windows.