crisgal, I know you got it from your local library but not all the public libraries use the same systems. If you tell me the name of the council concerned I can see what you need for your phone.
I was sure I posted yesterday - maybe I forgot to click 'submit' :(
Anyway, open the Overdrive app and see if there is the option to 'download' or 'add' books to the library. If so, click this and see if it finds the book on your phone.