It isn't there.
In theory, the Red Box characters should be able to transition directly classes from Heroes of the Fallen Land. In practice, several things got changed.
The Thief was among the worst. The thief as presented in Heroes of the Fallen Lands uses a basic attack augmented by various class powers and benefits instead of At-Will attacks.
I recommend rebuilding your character via the compendium. If you like using At-wills, then go with the Rogue from the Player's Handbook. There is nothing overly comparable to Defensive Strike though. Otherwise, the Thief is just fine, there is one in my campaign now and he does quite a bit of damage!
IMHO, your questions shows that Wizards of the Coast did not adequately plan the relaunch of the 4e line with the Red Box and Essentials. There should be no differences in the classes as presented by the two products.