It's been a long time since I used it, but I'll try to explain as best I can recall.
First, remember that mailboxes all really live on the actual mailserver, not in squirrelmail or roundcube. Both squirrel and RC store information about users separately, most importantly in this case the contacts and identities.
The plugin runs once per new user. When a user who has never used RC before logs in, the plugin attempts to gather the identity and contacts from squirrelmail and populate it into RC. If the user already has account info in RC (logged in prior to the usercopy plugin activation) nothing will happen.
To make it work, you need to activate the plugin, and also to edit the config.inc.php file inside the plugin to tell it where to look for the squirrelmail data. squirrel can store data in flat files, or a database. Open the config.inc.php.dist file inside the plugin folder to see the options you need to set.
