My husband is a GM employee and we have used this benefit many times. If you are there M-F during regular business hours (I think it is 9 - 5 as previous poster mentioned, but I am not positive.) behind the fast pass machines, as you are looking at the entrance, is the door you will use. Since it is your first time and you are not sure where to go, if you go in the exit, to the show room area, there is an information desk with cast members. They will be happy to show you where the door is. When you go through the door, you are in a hallway. At the end is a speaker and the elevator doors. Someone will say hello and something like please display your employee ID to the camera. (My husband's employee ID has a photo of him on it.) After they look it over, they will welcome you and open the elevator. You will ride up to the lounge. Follow the hallway around to the desk. They will have you sign in and then explain about the free Coke beverages. There is a small room with a couple tables, the coke machines and a glassed wall to the ride. It is dark in that section of the ride and hard to really see it well. Usually they will let you linger as long as you like. Sometimes if another party is coming up or they are busy, they will ask you to head on down to the ride. There is also a much larger area behind the welcome desk with chairs and a wall of windows that lets you view the area just outside the attraction entrance. (It does not face the direction of the fireworks.) You can see spaceship earth and there are some cute photo ops of your family holding up the "globe". There are also soft chairs and some large stuffed Mickey and Minnies. Also good photo ops. They also have very nice and clean bathrooms. When you are ready to ride, you let them know at the desk and they will either give you a card, or escort you to the ride. It is a bit of a maze down to the ride. (I assume that is how they backdoor the famous people.) When you get to the loading area a cast member will meet you and put you on the next car. It is really a nice perk. We enjoy getting away from the crowds and catching our breath, or enjoying the air conditioning for a few minutes. We usually make a plan of attack by spreading our park maps on the tables and deciding what to do the rest of the day.
If, you are there after the lounge is closed, or on a weekend; go in the exit to the information desk. They will check your ID and have you sign in there. They will then take you to the front of the ride. You will not get to go to the lounge, but you still get to front line the ride.
I am not sure what constitutes a GM employee. I would think a retiree would be fine, just don't forget a GM ID. The person on the ID must be present.
Have a great time at Disney and enjoy Test Track.