Whitelisting VST instruments
You must whitelist any VST 2 instruments you want to use in Dorico Pro. Because whitelisting works like a preference, you must only whitelist plug-ins once for them to be available in any project.
A default vst2whitelist.txt file is included with your Dorico Pro installation, which lists VST 2.x plug-ins that Steinberg has qualified for use with Dorico Pro.
You can create a second vst2whitelist.txt file in a user-specific location that is not overwritten if you subsequently update or reinstall Dorico Pro.
When Dorico Pro starts up, it reads both the default whitelist file and your user-specific whitelist file to build up the list of whitelisted plug-ins.
Prerequisite
You have quit Dorico Pro and all other programs.
Procedure
Result
When you next start Dorico Pro, it considers your whitelisted VST plug-in entries, making them available for use in the program.