If you've ziped up the entire folder, you may well have done it in the wrong order as Doitsu suggests.
Creating the files as you've done is good but I'd suggest instead of zipping up the folder, create an empty zip file and then drag the mimetype file into it as the first file. This file must not be compressed and has to go in before any other content. It also must only contain the following:
Code:
application/epub+zip
You can't put anything else in it, no other characters, carriage returns, etc.
Then you can added the other files with the compression now.
Have a look at the wiki here regarding epubs, that might give you the help you need. I've included a link to it below.
EPUB