3d party Barrel - can't get it to work

I want to use this barrel in my App project: https://github.com/douglasr/msgpack-monkeyc

My IDE is VSCode on MacOS, current version both for VSCode as for teh Monkey C extension. Git installed and working.

This is what I did:

1. Cloned the GitHub repo to my local folder.

2. Built the barrel in VSCode: Monkey C: Build Current Project. This resulted in a .barrel and a .barrel.debug.xml file in the /bin directory.

3. Ran the test in VSCode: Monkey C: Run Tests. This worked ok.

4. Opened my App folder.

5. Added the barrel to the project with Monkey C: Configure Monkey Barrel, selected the .barrel file

6. Added a call to a function in the barrel as per the instruction in the README

But, no matter what I try, the app does not compile. The compiler complains: "edge1050: Undefined symbol ':pack' detected." referring to the source code line in my app where the function is called:

var packed_poiSet = MessagePack.Serialize.pack(_poiSet);

Also there's a lot of similar errors, all refering to code lines in the test source.

I followed all instructions as docuemnted by Garmin and the author of the barrel to the letter, but it does not compile.

Obviously I am missing something which is not documented. Or maybe it is but I missed it.

Any help is appreciated.