In 3.0 products, namely Monster Manual 2 and Savage Species, from which the web enhancement Thri-Kreen came from I think, the race has a +1 Level Adjustment (in addition to their +2 ECL from racial hit dice), or a +3 Level Adjustment if they have natural psionic abilities (again, in addition to the racial hit dice +2 ECL). So total ECL of +3 if nonpsionic, +5 if psionic. Note that simply gaining levels in a psionic class does not matter, it's just whether or not they have racial psionic abilities.
Thri-Kreen are not in the SRD because they are Product Identity for WotC; they're like Illithids and such, in that they are not open content for other companies to use. Athas.org is the official Dark Sun website and has permission to use Dark Sun material like Thri-Kreen, but no one else can commercially use Thri-Kreen besides Wizards of the Coast.