Collaborative editing was introduced in Confluence 6, it can't be added to a 5.10.x instance.
The current version is Confluence 6.5.1, so there have been features introduced and bugs fixed since Confluence 5.10.x. Please review Confluence 6.0 Release Notes and Confluence 6.5 Upgrade Notes and test the upgrade in a non-production environment.
To add to what @Ann Worley wrote...
If you currently access your Confluence instance over SSL and are not doing it via a reverse proxy, but instead through configuration in the Tomcat server.xml file, you will need to change how you do SSL termination to use a reverse proxy instead. Collaborative editing will not work otherwise. If you don't care about SSL, then you don't need to reverse proxy.
Reverse Proxy Setup
Please see the diagrams at:
If you don't run Confluence behind a reverse proxy, you'll connect to Synchrony via Confluence's internal Synchrony proxy. SSL, if used, is terminated at Tomcat. This is the default configuration, and you shouldn't need to make any additional changes to use collaborative editing.
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