Plesk gives following error while installing modules which requires mcrypt to be installed.
Cannot load mcrypt extension. Please check your PHP configuration.
Install mcrypt using following steps on plesk.

1) download and install repository
wget -q -O - http://www.atomicorp.com/installers/atomic | sh
2) check the version of php

php -v
If php has latest version then install the mcrypt with following command

yum install php-mcryp
t

If php has older versions then compatible modules is to be installed. Under this situation
following command can be used to install mcrypt.

yum install php-mcrypt-5.1.6
(for php version 5.1.6)

3) restart httpd

/etc/init.d/httpd restart
4) restart plesk

/etc/init.d/psa restart