Many Opera users spent the summer of 2009 hopping all over the place trying to follow where Opera was putting their bookmarks.
First of all, launch Bookdog and in the Bookdog menu click Check For Update and update to Bookdog 5.3.6. Bookdog 5.3.6 accomodates Opera's wandering bookmarks by checking all three of the following locations and opening whichever has been modified most recently:
Opera 10 RC (Release Candidate) bookmarks:
~/Library/Preferences/Opera Preferences/bookmarks.adr
Opera 10 beta bookmarks:
~/Library/Preferences/Opera Preferences 10/bookmarks.adr
Opera 9 bookmarks:
~/Library/Preferences/Opera Preferences/Bookmarks
I have been told that there is an importer in Opera 10 for Opera 9 bookmarks. However, if you missed this or need to do it manually for some reason, you can rename the older "Bookmarks" file to "bookmarks.adr" and move it in "Opera Preferences", because the files are the same format inside -- just a different name.
You might want to tell Opera Software that the next time they rev their product it would be nice if they didn't move and rename the bookmarks file for no apparent reason.