First, your question might better have been asked on netbsd-help.  But we can
answer it here on netbsd-users.  (^&

I guess that when people say "use mkisofs", they are depending on you
knowing where to get it---or they come from a LINUX background where
everything conceivable was packed onto 10 "distro" CDs, and they simply
assume that if software compiles, it's included in the "base system".

mkisofs is not part of NetBSD.

It is available from pkgsrc.

It used to be available only in its own package,

Now, I think that it is (only?) available as part of the cdrecord
package.  Check out ...pkgsrc/sysutils/cdrecord.

-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/