Options=Preferences=Settings
for me
(In Fennec 2 Sync is included into browser, and there is textbox Options->Sync->Secret Phrase; but it's alpha, and crashes frequently, at least for me; so please, don't try installing this alpha until you have all the bookmarks backup-ed)

Where did you enter Secret Phrase into Fennec when you synced Fennec first time? Try to look for it here.

Edit: I'm going to ask question on
http://groups.google.com/group/mozilla-labs-weave
about this problem.