In the menubar of Sunbirds main window go to "Tools" -> "Extensions".
In the Extensions window select "Install" and browse to OpsCalendar.xpi and press "Open" (Alternatively you can drag the xpi to the Extensions window).
Restart Sunbird.
Configure the server settings
In the menubar of Sunbirds main window go to "Tools" -> "!OpsCalendar Options".
In the opened settings window fill in the Server address (including http://) and your username and password.
Configure which calendar is an OpsCalendar
In the OpsCalendar Options dialog select the calendars for witch you wish to use the plugin. Note that changes to the calendar selection are instant and that if you have psTimesheet installed that only one plugin can selected a particular calendar.