I think it depends on what the "Text - Order String - Use/Turn On" field's value is under your custom ability properties. This is the OrderID of the action/spell you want to perform. If it is 'shockwave', for example, ordering the unit to "Unit - Order Your_Unit to Orc Tauren Chieftain - Shockwave Your_Point" will order your unit to cast the custom shockwave ability if he has it. Change the type of "Unit - Issue Order" if the ability is activated without a target, with a unit target, with a destructible target, with a point target, etc.
Hope this helps!