We have resolved the issue in the Ian support ticket. You can find information about the labels usage bellow, since it might be useful for others interested in this thread:
In order to use the labels, you should specify the type of the resource and the key of the resource of that type. For instance:
The Log out text label is registered under the
PublicControlsResources type (the resource class) and has a key
Note that after adding the label, you should restart the application whether the resource you are editing is not alredy edited - there is already a resource file (.resx) in the App_Data -> Sitefinity -> Global Resources. The resources are also being cached, so an immediate change to take effect, a restart is required, as well.