Modules are not a tab in the system settings. We are in the process of refactoring Group-Office and we plan to move it here. You can find it in Start menu -> Modules
In this screen you can install modules and configure module access. To install a module simply check the box in the row.
After install a permissions window will pop up where you should select the user groups you want to grant access. To change permissions later double click the row.
In the permissions window you can double click the level. You can choose “Use” which grants basic access or “Manage” for users that need administrative functions in the module.