That is a function of your e-Reader software. It is not part of any Acrobat specification.
I'm having the same problem. I am unable to see the cover within the Adobe Digital Editions reader.
An eBook may not have a cover or depending upon how it was created the cover may not display first.
It is all a function of how the content author mastered the content and piped out the eBook.
Regardless, as Michael posted, it not an "Acrobat" issue; particularly as Adobe Digital Editions is being used.
A post to the Adobe Digital Editions Forum may get some useful replies.
Are you wanting to add it within the ebook (PDF file) or a cover outside of the ebook? This might help, Over 180 ebook cover templetes that you can place out side of you PDF ebooks or within the file itself. Check it out ebook cover templetes
I had the same problem.
Thank you this works for me
If you want to put covert inside your eBook, its simple just create a cover in PDF editor and put them as first page of your PDF.