How can I create a new showreel?
In the menu, click on "Manage Media" and then on "Showreel Manager".

On the right side, above the video library, click on "+ New Showreel":

In the window that opens, you can enter the title for the showreel, provide additional details about its visibility, and upload a thumbnail image:

Please note:
If you are not a Premium member, you will have limited configuration options for showreels; advanced options such as thumbnails or the download option are only available to Premium members.