how can I share a photo stream from my mac? there's no check box in system preferences to turn it on

I've been having trouble to share my photo stream with other from my mac. Once I selecte the photos and click share>photo stream, nothing else happens. The option for "new photo stream" does not appear, neither the field where you type in the email addresses from people you whant to share it with. I all ready checked in iphoto'11> system Pref.> photo stream and there's no "shared photo strem" check box to turn on or off... I also checked in my mac system pref to be sure that my icloud is enabled and it is.

If I select photos and then go to my tool bar and press share. the option of photo stream is not available.

I also can't accept any shared photo stream from other and view it in my iphoto. I can only access it via web.

I DO can use my photo stream correctly and share it via my iphone. and every picture that I take uploads into my mac and every photo I import from cameras into my iphoto also uploads fine into my Ph stream. THE PROBLEM IS THAT I CAN'T SHARE THEM FROM MY MAC.....

Please can someone help me?