Professor Kukui adds 20 damage to the Active Pokemon during an attack. Does this still occur if you attack a benched Pokemon? I know it shouldn't be added on if you poison the Active Pokemon as this is an effect of an attack.
Just in case you have a different misunderstanding than BJ's answer covers:
If you do damage to a benched Pokémon with an attack that does damage to both Active and Benched Pokémon, then the Active Pokémon takes 20 more damage. For example, Inteleon SSH58 does 120 to the Active and 20 to the Bench; with Kukui it would do 140 to the Active and 20 to the Bench.
If you do damage only to the benched Pokémon, then neither is that damage increased nor do you do 20 damage to the active Pokémon when Kukui is played. From the Compendium meta-rulings section:
An attack has to do "some" damage before an effect can add "more" damage to it. (Jan 31, 2008 PUI Rules Team)
So for example if you play Tapu Koko SM20 and use its Flying Flip attack, which does 20 to each benched Pokémon but no damaage to the Active Pokémon, it would not increase that.
And to be extra clear; damage counters or other effects are not the same as damage to the Active Pokémon. If you planed Mew SM215, and used Psypower, and put all 3 damage counters on the Active Pokémon (or any other number), Kukui would not affect that. See the Compendium again for that, the next meta-ruling down:
Placing damage counters is not the same as doing damage. Things that add to or subtract from damage done do not affect placing counters. (Jan 31, 2008 PUI Rules Team)