I frequently see tips for doing things that require launching the Notes client on the server. If you're running Domino on NT that works great. But if you're not running Domino on NT (which is becoming ever more popular), that's probably not an option. It takes a few more steps, but you can still get around the ACL security.
I'm primarily running Domino on iSeries (AS/400) servers so this tip is based on that platform. However, it can be applied to other non-NT platforms as well.
Note: If "Enforce a consistent ACL across all replicas of this database" is set in the database's ACL, you cannot modify the ACL by opening it "locally" unless you're already in the ACL as Manager, regardless of the platform the Domino server is running on.
In order to do this you will need to have the ability to: Get and put files on the iSeries via FTP and delete a database off the server.
I begin by checking the "ServerStatus" tab in the Admin client to make sure nobody has the database open. Then, from a workstation with a Notes client, use FTP to get the database local. Then, using a terminal emulator, delete the database off the server.
On the workstation, launch the Notes client and open the database "locally" and add yourself, or your admin group to the ACL as manager. Then FTP the database back to the Domino server's data directory.
I issue the Domino console command "dbcache flush" after deleting the database off the server and before using FTP to put it back. Also, on the iSeries platform, after using FTP to put the database back in the Domino server's data directory, you need to change the owner of the object to "QNOTES" (The AS/400 command is:
CHGOWN OBJ('/path/database.nsf') NEWOWN(QNOTES)
where "path" is the full path to the domino server's data directory and "database.nsf" is the database's filename.