As Russ said, the version on /n/sources is a 4th edition version.
You must have the 3rd edition version off his website.

The real source is in /acme/bin/source/acd.

If it isn't there, then run replica to update your source.

If you can't do that, then read the manual page for fmtinstall(1)
which gives a very good example of how this stuff works.

If you can't do that then

int
msfconv(Fmt *fp)
{
	Msf m;

	m = va_arg(fp->args, Msf);
	fmtprint(fp, "%d.%d.%d", m.m, m.s, m.f);
	return 0;
}

is what you need.