Going back to your answer to my post.
You tried doing usermod -G wheel -a martin, logging out and logging back in?
Pretty much everything else is overcomplicating it.
Please try doing your su to root then
grep wheel /etc/sudoers
and post the result.
If you get back the line I mentioned about %wheel ALL=(ALL) ALL
Then please type
If you are in the group wheel, all is good. If you aren't then I suspect you mistyped the command I gave you.
There are various possibilities, but I suspect that the results of those two commands will help us clear things up.