[Deceptor]

Make sure you've configured your memcache settings in includes/config.php for vBulletin, although you don't need to connect it to the vBulletin datastore vB Optimise needs to be able to read the connection information.

If that's done, check that you've spawned an instance of memcached on your server, simply installing that and the memcache extension for php doesn't run the server. For example, running this in SSH will spawn an instance with 128mb of memory available, 1024 max connections and under the user "nobody" and port 11211 which is typically the default:

memcached -d -m 128 -u nobody -c 1024 -p 11211