Thanks for your support, it's highly appreciated.
To create a preset:
_ I select the Forest Pack object
_ I copy it in a new empty scene (I use a Copy/Paste script found on scriptspot.com)
_ I save the scene in the "objects" folder of the chosen Library folder
_ I collect and relink all the assets
_ I create a thumbnail
_ I create the item in the Library by doing: Create item > Create 3D Object
_ I manually fill the necessary information in the "Properties" of the item
I have to say your method is way faster and convenient, thanks for that.
So I followed your link and created the preset using "Import Objects from Scene..." but it gave the following error:
"Scene File Save Warning: Unable to rename temporary scene file E:\My_Itoo_Presets\...........\objects\xxxx.max.sv$ to scene file E:\My_Itoo_Presets\...........\objects\xxxx.max"
So I hit OK and visibly the process didn't come to an end as I found a temporary file with a .max.sv$ extension instead of the file name I filled in the "Import Objects from Scene..." dialog box.
At that point I renamed the extension of the temporary file to .max, so that it can be found by Forest Pack, and I opened a new empty scene and tried using the preset: it failed with the exact same behavior as I first described:"Warning: can't find object "" in the source scene" + no objects were generated by the Forest Pack object.
PS: I double check I could open the .max: it's working fine. I can modify the Forest pack object, it's rendering well.