Good day today, first I went down a wrong path, I found some websites, that lead me down a road of copying a bunch of files from /os2 to /os2additions.
I guess that was for old versions of virtualbox, I'm using 7.0 release something or other.
Then I found ReadmeArcaos in c:\os2additions directory, lots of good information in the readme on vboxcontrol and the vboxservice
At first I was going to do like windows and map under c, but then I found, I could map a whole new drive letter (d:) which I did and it worked great.
I copied the watcom installer (from 88watts) to the shared folder and ran it.
Then I was able to compile and run mthread.c from the os/2 sample directory.