I disagreed with the ruling, pointing out that the ETB trigger did not activate an ability, it just triggered one, and that an activated ability is one with "cost: ability". The judge's ruling held and I missed an opportunity to swing lethal because of it.
Correct. I have actually read the rules on the wizards website. To copy an ability. As Illusionist bracer does it must be able to produced that on the stack. As a trigger resolves at instant speed and doesn't enter the stack illusionist bracer is unable to interact with them (that's how I interpreted the rulings anyways).
Also an activated ability is identified with a cost followed by : (Colens) if it doesn't it's a triggered effect and illusionist bracer has no way copying its effect
As a trigger resolves at instant speed and doesn't enter the stack illusionist bracer is unable to interact with them
This is wrong. Triggered abilities use the stack just like activated abilities. The only reason Illusionist's Bracers don't work with triggered abilities is because they trigger, rather than activate. From the Gatecrash FAQ entry for Illusionist's Bracers:
Quote from Gatecrash FAQ »
An activated ability is written in the form "Cost: Effect."
if it doesn't it's a triggered effect and illusionist bracer has no way copying its effect
Also not quite correct. For an ability to be a triggered ability, it must start with the words "when," "whenever," or "at." They are written in the format "[Trigger condition], [effect]."