Attack Flying Units

I wan't the unit under the Winged Sliver buff to be given the ability to attack air units. I cant figure out why this won't work.

Sliver Tower
Events
Time - Every 3.00 seconds of game time
Conditions
Actions
Unit Group - Pick every unit in (Units in (Playable map area)) and do (Actions)
Loop - Actions
If (((Picked unit) has buff Winged Sliver) Equal to True) then do (Unit - Add Able to attack flying units classification to (Picked unit)) else do (Do nothing)

I know that the unit sucessfully recieves the buff but the unit still can't attack air units. I know that there is another way to do this by creating a second attack that can attack air and having a trigger enable it but doing it that way seems much more time consuming.

I don't think the classification actually changes the attack, it's just a classification, like the "Aincent", or the "Sapper". It may have an effect in an obscure part of gameplay, or it may be something left behind from the development of warcraft when they thought they would need it, who knows...

I think enabling the second attack is your best option.

give them the orb ability without takin an item slot.

Could sombody please explain this a little more clearly, thanks.

Unit - Add Ability : Orb,
also I prefer to change item abilities to unit abilities before using them with units, just go to ability editor and set "Item Ability" to false.