Cannot find VST instruments

I am quite new to Cubase, but are familiar with other MIDI/recording software. I bought a Steinberg UR 242 Sound card. It works fine with the boundled Cubase AI / LE 9, but when I installed some new VST keyboards, they don’t appear as new synths. They are located in plugin manager, and are in 64-bit (Windows OS).

Some examples of installed VST’s:
http://www.vst4free.com/free_vst.php?id=398
https://www.tunefish-synth.com/index.php/download
https://asb2m10.github.io/dexed/
http://www.audjoo.com/h/Helix.htm
http://www.geocities.jp/daichi1969/softsynth/

When installing a mix plugin, it disappears in the menu right away.

Is this a limitation of the Cubase AI/LE or is there something I have missed?

Hi and welcome,

Make sure the plug-ins (*.dll or *.vst3) files are stored in the scanned folder, or set Cubase to scan the folder, where is the plug-in stored.

Make sure the plug-in is not Blacklisted.

Then you should see the plug-in. Can you see the plug-in in the VST Instrument list?

No, there is no Cubase AI limitation regarding to plug-ins. You should see all plug-ins, same as it is with Cubase Pro.