You only add the Effect to the damage roll.
There is no additional damage for scoring a critical hit on an opponent.
If you attack an opponent (creature, person, alien, robot etc.) directly then the only "critical hit" feature is that you always inflict at least 1 point of damage if your attack has an effect of 6+. I knew this rule existed but it took me a while to track it down. It's under the Armour sub-header in the Damage section (MGT 2E Core Rulebook 74):
Armour reduces the damage sustained in an attack by the value of its Armour score. An attack with an Effect of 6 or more always inflicts at least one point of damage, regardless of the actual damage rolled for or the Armour score.
Critical Hits Apply to Vehicles and Spacecraft
Only vehicles and spacecraft have detailed critical hit rules. You can find these rules on pages 134 (vehicles) and 158 (spacecraft). For vehicles (MGT 2E Core Rulebook 134):
If an attack roll against a vehicle has an Effect of 6 or higher and it causes damage (rather than just bouncing off armour), a critical hit has been scored – some vital system has been damaged by the attack, reducing the effectiveness of the vehicle.
The rule for spacecraft is identical. Your craft can also suffer a critical hit if you skimp on maintenance, so don't let that fall by the wayside.
In each of the rulesets on vehicles and spacecraft is a table on which you determine which system was hit, with a 2D roll. The Critical Hit Effect that is inflicted to that system is determined on the Critical Hit Effect Table (Vehicles, 135 and Spacecraft, 159) by applying the severity of the critical hit (MGT 2E Core Rulebook 134):
The Severity of the critical hit is equal to the damage the vehicle has taken from the attack, divided by ten (rounding up).
Same goes for spacecraft.