Plex Home Theater is designed as a 10-foot UI and is thus intended to be controlled via a remote control or keyboard. Plex Home Theater does not currently support input or control with a mouse and there are no immediate plans for that to change.
If you wish to use Plex on your computer as part of your desktop experience, you can instead use the Plex Web App or Plex for Windows 8.1 apps, which have excellent mouse support.
Related Page: Plex Media Server > Using Plex Web App
Related Page: Plex for Windows 8.1
Related Page: Plex Apps > Plex Home Theater > Keyboard Shortcuts