Trouble downloading to my Sony E Reader

Hi, we have recently purchased an Imac and I have downloaded Digital Editions 3.  I successfully downloaded 4 new books from Waterstones into Digital Editions, but when I connect my E Reader, it does not come up in the Library section anywhere, to enable me to move the books onto it.  Can someone please help.  I am sure it is just a simple mistake I am making, but having looked through the help sections, it just says that the computer will automatically recogonise the e reader and it will appear in the library section.  Well, it isn't!  All help, very much appreciated, thank you.