Oh! My fault; I was confused!

If you have access to search another user's journal, it will appear as a link on that user's profile -- there's no way (yet!) to just search among your circle from the search page, and to search someone else's account involves a URL change.

For instance, check this: http://dw-suggestions.dreamwidth.org/profile

Last element of the interaction menu (on the left in most site schemes) is "Search For Entries". Clicking that brings you to http://www.dreamwidth.org/search?user=dw_suggestions

o Site Search (Public Entries) o Journal Search: dw_suggestions

So the journal being searched changes.