It's there under iCloud in system preferences for me, also on ML GM.
Set up Photo Stream on a Mac to work with iPhoto
Choose Apple menu > System Preferences, and then click iCloud.
Select the Photo Stream checkbox.
Open iPhoto, select Photo Stream in the Source list on the left, and then click Turn On Photo Stream (on the right).
To change when photos are uploaded to iCloud or imported into your iPhoto library, choose iPhoto > Preferences, click Photo Stream, and do any of the following:
If you dont want photos you take with your iOS device automatically imported into your iPhoto library, deselect the Automatic Import checkbox.
You may want to do this if youd rather choose which Photo Stream photos are imported into your library. If automatic import is off, your iPhoto Photo Stream displays the last 1,000 photos added to your Photo Stream. You can also drag photos from your Photo Stream to your library to keep them permanently.
If you dont want all the photos you import into your iPhoto library automatically uploaded to your Photo Stream, deselect the Automatic Upload checkbox.
You can always add photos to your Photo Stream manually.
For more information about Photo Stream in iPhoto, open iPhoto, and then choose Help > iPhoto Help.