You've purchased the Encrypted Database Extra and used the Register & Upgrade page to install it on your SimpleRisk instance.  You then click to "Activate" the Encrypted Database Extra, but you receive a message saying:

The "mcrypt" extension needs to be installed for the Encryption Extra to work properly.  Please install it to continue.

In order for SimpleRisk to properly handle encrypting and decrypting sensitive text fields in the database, it will require the PHP mCrypt extension to be installed.  Fortunately, with Ubuntu this can easily be done with a few short commands:

sudo apt-get install php5-mcrypt
sudo php5enmod mcrypt
sudo service apache2 reload

Once the PHP mCrypt extension has been installed, that error message should go away, and you should be able to successfully click on the "Activate" link to activate your SimpleRisk Encrypted Database Extra.