There are a lot of ways to do this in Notes 3.x, but here are a few:
1) If you have multiple servers, copy the database from one server to another. This will unhide the design in the new copy. The database will still show as hidden, so change the name to a template file(ntf) and create a new database from this template. This will insure the database is truly unhidden.
2) Make the locked database a design template. Create a new database. New database inherits design from template. Refresh design from template.
3) Modify a byte in the .nsf file; this is typically done to hack an application that was sold, so this technique will not be described here.
Lotus provides a database Hide Design tool (implemented in Notes 4.x and 5.x) that secures the database design more thoroughly. It is not possible to unhide a database design in these later versions of Notes without the original unhidden template; i.e., you can't unhide it by modifying a few bytes in the .nsf file. In addition, if the original design included LotusScript files from the file system when an agent or design element was saved, you'll need these files in the right directories to modify the corresponding agent or design element.