Please see the FAQ thread, which hopefully contains the answer to your question. You need to look at the knowledge base article about getting VST2 instruments to appear.
If you have Cubase, look in Vst2xPlugins Cubase.xml in C:\Users<USER>\AppData\Roaming\Steinberg\Cubase 8_64 (I don’t know where in Mac)
Search your loaded VST and copy whole code and put in Vst2xPlugin Infos VSTAudioEngine.xml in C:\Users<USER>\AppData\Roaming\Steinberg\VSTAudioEngine_64
Copy name and ID of your VST like…
play_VST_x64
565354706C6179706C61795F7673745F
Put in vst2whitelist.txt in C:\Program Files\Steinberg\Dorico\VSTAudioEngine\Components.
Put code…
Well, your VST plug-ins are in the right place, which is a good start, but now you need to ensure that the names and GUIDs for all of the plug-ins you want to use in Dorico into the vst2whitelist.txt file, which is described in the knowledge base article here.
I’m afraid I’m not sure what else to suggest at this point. You could try trashing the VSTAudioEngine folder in ~/Library/Preferences and then restart Dorico to see if that gives it a bit of a nudge.
Have you definitely correctly added the plug-ins to the vst2whitelist.txt file, and enabled them in the defaults.xml file, as discussed earlier in the thread?