To search, enter a word or words from the title of the manual you're looking for, or some letters or digits from the part number. Each word you search for must contain at least three letters (or you'd get loads of false hits from part numbers). When you're searching for part numbers, don't worry about the difference between '0' and 'O'; DEC frequently confused the two in part numbers, so they're treated the same when searching.
The search results consist of pages showing ten manuals at a time. The results show the manual's part number, date of publication, title and status, which is either blank or some combination of "Online" and "Superseded". Superseded means that a manual has either been revised since this edition or it has been replaced completely. Superseded manuals might still be online, so this catalogue still links to them. The manual that superseded this one will normally be found in the same search results.
The title of each manual will be a link if there is more information available. If the status is "Online", clicking the link will show you all the copies available. If the title is a link but the manual isn't shown as being online then at least the Table of Contents is available. This isn't too useful (or common) but if it appears that the manual contains information you would like, you can at least ask on appropriate newsgroups or mailing lists whether anyone has a copy.