Here's my setup;

5 Toons, Baylen, TheRuiner, Menage, Aahh, Trois

Character sets: 5 characters on 4 computers
  • Baylen-TheRuiner 1 computer
  • Menage 1 computer
  • Aahh 1 computer
  • Trois, 1 computer
Action Target Groups:
  • DPSShaman (Aahh, Trois)
  • Healer (Menage)
  • Deathknight (TheRuiner)
  • Shaman (Menage, Aahh, Trois)
When I created my buff key, I went to New Mapped Key -> New Step -> New Keystroke Action. In Target drop down box, you should see all your options that you've got setup. You can select character slots, names, or all of or other any of the groups you've got setup. If you're groups aren't showing up here, verify you've got the Characters assigned to the character sets.