As a Human Hunter with the Bonus At-Will Power option, can I take Twin Strike as an at-will power?
A Hunter is a type of Ranger, and the Twin Strike power is a Ranger at-will power. Hunters do not normally have any choose-able at-wills they can take Twin Strike with, but a human with the extra at-will can acquire Twin Strike that way.
You are correct that you are allowed to choose the extra at-will rather than take Heroic Effort, even if your character is an Essentials class. From PHB1 errata (warning, PDF!) p46:
When playing a human you can now choose between taking a bonus at-will attack power from your class or gaining the Heroic Effort racial power.
After some research, it appears that using the rules-as-written, you technically probably could.
A New System
However, even if the designers of Essentials were required to make it technically backwards compatible with the Player's Handbook (PH) character creation rules, they didn't make much of an effort and it's clear that the Essentials rules are intended to be a new character creation system with new mechanics.
Those new mechanics involve asymmetrical classes which gain different numbers and types of features and powers at each level. As part of this, not all classes have a long list of At-Wills to choose from at level 1, and therefore the PH's Human Bonus of an extra 1st-Level At-Will no longer makes sense. The designers recognized this, and introduced a new version of the Human with a racial ability called Heroic Effort to replace the At-Will.
Therefore, since you don't get an extra at-will, you can't take Twin Strike.
But Still the Same Game
That being said, Oblivious Sage has referenced errata published for the PH which is intended to take the Essentials rules into account, and could be interpreted as allowing you to stick with the extra at-will, even if you're creating a Human using an Essentials class. In which case, yes you can take Twin Strike.
Just Ask the DM
Different DMs might approach the interaction between the original PH character creation rules and the Essentials character creation rules differently. Just ask your DM and do whatever he says.