For those who don't know where the Westin is, here is how to get there. Go to the FRONT entrance of the Hyatt. This is NOT the side that features the heinous stairs, the brand-new Habitrail, and the swimming pool...this is the entrance that lets out onto Peachtree Street.
Exit the Hyatt and turn left. Go 1 1/2 blocks. As you proceed down Peachtree, on the opposite side of the street from the Hyatt, you should be able to spot the large "WESTIN" sign. You will pass the MARTA entrance to Peachtree Center and the Food Court, and the Hard Rock Cafe. In the next block, the Westin entrance is directly across the street from Hooters, a pizza place, and a still-to-be-built BBQ joint. It is also next to Fire of Brazil (yay!) on the same side of Peachtree. There is also Meehan's Pub (even more yay!), a bit further down the block.
As you go in the front entrance of the Westin, the Alt History Track Room (International C) will be on your right and the Whedonverse Track Room (International D) will be on your left after a short flight of stairs.
(EDIT: According to sdrg below (thanks!), the handicap accessible entrance is on the right side of the hotel facing Peachtree Street, that is, on the side closest to the Hyatt. There is currently some scaffolding there due to renovations.)
As for the Westin Ballroom, where several AH and Whedonverse track functions will be held, it is on the 8th floor, easily accessible by elevator. There is also a balcony level, accessible both by spiral staircase from the Ballroom pre-function area and also by elevator to the 9th floor.