What qualifications are required for an authorized flight instructor to teach IFR in AATD simulators, in the U.S.?
To provide instrument instruction the instructor must posses a valid CFI certificate with an "instrument airplane" rating.  This is irrespective of the type of training device or airplane used with the caveat the instructor needs to be rated for the airplane (either flown or being simulated).  In other words, if you cannot instruct in an airplane, you cannot instruct in a training device either.

The applicability of using the AATD is more of a program certification than an instructor pilot certification issue as there are different levels of training and special certifications for certain levels of training devices and simulators.  See 14 CFR 61, 141, 142 for details about training programs and AATD use.