Your Opera bookmarks (.adr) were added to Netscape bookmarks (.html), you only enable Netscape bookmark plugin in Edit -> Preferences -> Plugins.
(Restart KM can be needed)
Bookmarks menu will be showed in menu bar, your Opera bookmarks must be in a bookmark folder named Imported Bookmarks. You can use Bookmarks -> Edit ... to move or reorder your Opera bookmarks.
K-Meleon in Spanish