Ok, found the folder. I also made my User Library Folder visible (in Finder) using the Terminal commands you posted. However, the User Library folder remains invisible inside GB even if it is visible in the Finder!
Do you use the "load fxp file" in the Preset Menu of the PT Plugin (within GB) to open your Presets, or the "Load/More" Command?

Indeed the "More/Load" and also the "Load an FXP file" from the the Pianoteq menu both open a file chooser that does not show the invisible files even if the Finder by itself does outside of GB. I never use these, I simply select the preset from the menu or in the preset manager.

I guess these commands are for loading fxp files that are outside of the Modartt structure and in a visible directory.

Yep! However, after copying my Presets to the "deeply nested folder" where GB stores its Pianoteq Plugin presets (i.e. Library/Audio/Presets/Modartt/Pianoteq6), I also had to change the .fpx extension to .aupreset
But it works! Merci bien Gilles!!!

Pas de quoi!

Maybe something changed in GB 10.3.2 but on mine the "deeply nested folder"  for user presets is actually quite long:

"/Users/gilles/Library/Containers/com.apple.garageband10/Data/Library/Application Support/Modartt/Pianoteq/Presets/Mes presets"

and it contains normal .fxp files. Maybe you chose the wrong folder?

merci Gilles, but...do you have any idea why my Preset .fpx files appear greyed out  in the Pianoteq  Plugin inside GB?

Gilles wrote:

aWc wrote:

I am glad I found this old thread: 
https://forum.modartt.com/viewtopic.php?id=6547
that helped me finally make the Pianoteq (Pro 6.6) Plugin work inside Garage Band (10.3.2). However, I can only access the factory Presets.
In Reaper and other software I've tried, I see my saved Presets in the Plugin just like in the Standalone version.

There is the option, in the Plugin,  to "Open fxp file...", but as the fxp files are located in the User Library folder... Mac users know that this Folder is "invisible" except from the Finder (by using "Go to...", or holding the "Alt" key to make it visible in the Menu)...an extremely irritating thing !@#%$*&!

Any thoughts?

UPDATE: I found the option (in the Preset menu of Pianoteq running as a Plugin in GB): "...More" , and then "Load...", which indeed gives access to the User Library/Application Support/Modartt/add on/Presets/My Presets.....BUT they are greyed out..:(
Why?

You can make the "invisible" files permanently visible with these two commands in Terminal:

defaults write com.apple.finder AppleShowAllFiles TRUE
killall Finder

Then restart Finder by clicking its icon.

I just tried GB version 10.3.1 under Mojave on my MacBook Air and the fxp in the preset manager or the menu are only the factory ones. I remember Julien telling me that GB has its own space for presets and the ones created outside GB are not directly accessible. If you create a new one inside GB, you can get the name of the deeply nested container directory where GB keeps it by clicking on the folder icon at the bottom of the preset manager and copy your older presets there.

I am glad I found this old thread: 
https://forum.modartt.com/viewtopic.php?id=6547
that helped me finally make the Pianoteq (Pro 6.6) Plugin work inside Garage Band (10.3.2). However, I can only access the factory Presets.
In Reaper and other software I've tried, I see my saved Presets in the Plugin just like in the Standalone version.

There is the option, in the Plugin,  to "Open fxp file...", but as the fxp files are located in the User Library folder... Mac users know that this Folder is "invisible" except from the Finder (by using "Go to...", or holding the "Alt" key to make it visible in the Menu)...an extremely irritating thing !@#%$*&!

Any thoughts?

UPDATE: I found the option (in the Preset menu of Pianoteq running as a Plugin in GB): "...More" , and then "Load...", which indeed gives access to the User Library/Application Support/Modartt/add on/Presets/My Presets.....BUT they are greyed out..:(
Why?

You can make the "invisible" files permanently visible with these two commands in Terminal:

defaults write com.apple.finder AppleShowAllFiles TRUE
killall Finder

Then restart Finder by clicking its icon.

There is the option, in the Plugin,  to "Open fxp file...", but as the fxp files are located in the User Library folder... Mac users know that this Folder is "invisible" except from the Finder (by using "Go to...", or holding the "Alt" key to make it visible in the Menu)...an extremely irritating thing !@#%$*&!

Any thoughts?

UPDATE: I found the option (in the Preset menu of Pianoteq running as a Plugin in GB): "...More" , and then "Load...", which indeed gives access to the User Library/Application Support/Modartt/add on/Presets/My Presets.....BUT they are greyed out..:(
Why?