If you're running b2e on OS X (I'm probalby the only one :P) you will probably run into a problem where "mkdir cannot create a directory.. bla bla bla.. media/users"

Anyway, the problem is simply that you probably installed b2e as an admin-level user, and Apache/PHP runs as 'www' which is not an admin user.

Simply change the directory permissions.

sudo chown -R www /path-to-your-b2e/blogs/media