As Noah says...

Once the macro is set (you see it scroll by on the alts screen), you need to press the keybinding associated with the raid icon to make the macro fire. Have a look in your keybindings for a heading called Jamba Target.

  • Target 1 (Star)
  • Target 2 (Circle)
  • Target 3 (Diamond)
  • Target 4 (Triangle)
  • Target 5 (Moon)
  • Target 6 (Square)
  • Target 7 (Cross)
  • Target 8 (Skull)

So if you mark a skull; you need to press the key your bound to Target 8 (Skull) for your toons to target that mob.

You can change the macros so that they do other things as well as target, like sheeping for instance.

Hope that helps, please post a reply if it doesn't.

Also, if there are multiple mobs with the same name, your toon will target the closest one (not necessarily the one that is marked). This is (unfortunately for us) a part of the game that cannot be changed.