No option to create shared photo stream?

I'm using Aperture 3.4.1 on both my iMac and my MacBook and am unable to create a shared photostream on either.

According to the instructions at http://help.apple.com/aperture/mac/3.3/#aprd8aa8a57, the user is supposed to:

To create a shared photo stream

1. Select the photos you want to share, then do one of the following:
   • Choose File > Share > Photo Stream.
   • Choose Photo Stream from the Share pop-up menu in the toolbar.
2. In the Photo Stream dialog, choose New Photo Stream from the Photo Stream pop-up menu.
3. In the To field, type the email addresses for your invitees., etc.

However, I never receive a Photo Stream dialogue or an option to choose "New Photo Stream". When I click on PhotoStream from File>Share, Aperture immediately begins uploading the photos to my main feed. Occasionally there is a flash that makes me wonder if Aperture is somehow suppressing a dialogue box.

Thanks for your thoughts and suggestions.