Hi,
I would say svennilenni was right. As I can see, this is plug-in collection names VST Effects, which is not the default one. The default collection, where you can see all plug-ins is named Default.
On the right side of the plug-in list click to the arrow (Plug-in Collections and Options) and switch to the Default plug-in Collection. Or add your External Effect to the VST Effect collection in the Studio > VST Plug-in Manager.