by Andrea Bicciolo.
From the screen shot provided looks like you do not have any additional plugins, only the default ones are available.
If you are running a linux box, there are very good chances your distribution already has a memcahced package, for example on a Debian 6 "Squeeze" box you can just install and start memcached with a default configuration using the following command:
# apt-get install memcached
Then you need to install php5-memcache or php5-memcached modules, and also in this case most linux distros provide their packages. Once memcached and related php module are both installed (remember to restart your web server), access again the Moodle cache configuration page, you should see a green check mark and the "Add instance" link for the corresponding memcache or memcached plugin.
If you are running a linux box, there are very good chances your distribution already has a memcahced package, for example on a Debian 6 "Squeeze" box you can just install and start memcached with a default configuration using the following command:
# apt-get install memcached
Then you need to install php5-memcache or php5-memcached modules, and also in this case most linux distros provide their packages. Once memcached and related php module are both installed (remember to restart your web server), access again the Moodle cache configuration page, you should see a green check mark and the "Add instance" link for the corresponding memcache or memcached plugin.