I installed the mcrypt library, but for some reason Apache disregards it. The files are in place, reinstalling invokes messages that newest version is already installed.
Have you tried restarting apache after installing?
Edit: Yes, that's how I would restart it. Also FYI, after installing the mcrypt extension, you must then recompile php with the --with-mcrypt=/usr/local/bin/mcrypt option, except use the path to where the mcrypt extension is installed.
There are two individuals in the comments on this page who experienced the same error. Might try their suggestions: http://us3.php.net/manual/en/mcrypt.setup.php
Also, note the comment here regarding the same error: http://us3.php.net/manual/en/mcrypt.installation.php
Which platform do you use? Are you using a Mac? Installing an Apache module on a Mac can be a tricky process.