Follow the steps for installing a plugin as they are described at http://pkp.sfu.ca/ojs/

It may be beneficial to create a tar.gz of this plugin and use the web tool for installing it.  
Otherwise, you will need to insert the correct reference into the versions table.

Alternatively, you may download a version of the plugin here:

http://etcdev.hil.unb.ca/jason/statisticsAggregator-2.2.tar.gz

This version is designed to work with OJS 2.2. If you want the version for OJS 2.3.x, please visit

https://github.com/jnugent/OJS-Statistics-Aggregation