Q: I cannot enable Photostream on my iBook though it's active on my iPad - any ideas out there please?

See  http://www.apple.com/icloud/setup/  for instructions of setting PhotoStream up

See http://support.apple.com/kb/HT4486?viewlocale=en_US&locale=en_US  for FAQs and trouble shooting links

LN

If your iBook supports Mt. Lion and iCloud and you are correctly signed in to the same iCloud account on your iBook that the iPad is signed into try the following:

Disable Photo Stream in the System/iCloud preference pane

and in iPhoto's Photo Share preference pane.

Reboot and reenable both respectively. That should jump start Photo Stream.

NOTE: You mention iBook in your post but your profile indicates MacBook.  Which is it an is it running Mt. Lion?

OT