So Tessa Thompson is gueststarring role starts on episode 6 this season. So the date is October 26. So make sure to mark the date down.

Heroes – Episode 4.x6 – Strange Attractors – Official Synopsis

10-26-2009 8:00PM

THE HEROES ATTEMPT TO RESCUE THEMSELVES AND OTHERS WITHOUT THE USE OF THEIR POWERS

Matt (Greg Grunberg) goes to extreme measures to extinguish Sylar (Zachary Quinto), who continues to torment him. Elsewhere, HRG. (Jack Coleman) calls in a favor from Tracy (Ali Larter) to save a troubled young boy whose misfortunes resemble her own. While Claire (Hayden Panettiere) and Gretchen (guest star Madeline Zima) are being hazed by their new sorority sisters, they realize pledging might not be their biggest problem. Robert Knepper also stars. Lisa Lackey, Tessa Thompson and Rick Worthy guest stars.

EDIT: Her first appearance is on Heroes 4×05