There are two types of pending scenes:
The "Pending scene(s)" box is only visible, if you have at least one scene invitation.
Scenes you were invited to
You can accept or reject the invitation by clicking on the scene's name and after that on the "Accept invitation" or the "Reject invitation" button.
Scenes you have applied to
The administrator or the professor in the scene can accept or reject your application. If your application gets accepted, then the scene will appear among your scenes (on the Main page). You can set an email notification about about the acceptance or rejection of your application.