I mark my posts with “featured” (without the quotes) and posts still do not show.
Firstly, ensure that the posts are correctly tagged “featured” without the quotation marks.
Next, delete any previous category called featured you may have had before installing the theme.
Lastly, ensure that the ‘tag slug’ for the featured tag is actually set to “featured” and not “featured-x”. You can edit the post tags by navigating to Admin > Posts > Post Tags