This is just another issue with SharePoint Online specifically.
Please see this forum:
Missing Property Query option in Scopes
On June 15, 2012, a Justin Ronan from MSFT support stated:
This is Justin with Microsoft Office 365 SharePoint Support. Thanks for posting your question on the community.
I apologize but we have recently found an issue with our search scopes system and have disabled the feature. We are currently working on the fix, but until completed, site collection administrators will be unable to create, edit, or delete any of their search scopes or display groups. At this time there is no ETA for the fix and only scopes created in the past prior to the issue will work. Keep an eye on the Release Notes and Official Blog for updates or known issues to the product. We apologize for any inconvenience this may cause.
Thanks and Regards,
Microsoft Office 365 SharePoint Support
Also, Wictor Wilen (MVP) addresses this in his blog post You cannot create property based search scopes in Office 365 (SharePoint Online)