It's in the middle of the terminal. Southwest is near the end, so just go down that one level to the rental car level, then you walk to approximately the middle of the terminal, cross a street and
Alamo is on the ground level and left side of the parking garage. 5 minute or so walk...(much easier if you have wheeled luggage or get one of those carts...and do the online checkin)
As far as the free driver goes, try to find a discount code that includes the free driver...and make sure you do the "paperwork" when you do the online checkin. You'll also need to show both licenses when you exit. Costco is a good one for including a free driver, and I'm sure there's others. But make sure you qualify...for instance, if you use Costco for the additional driver and you're not a Costco member, there's a very good chance you'd have no problem renting the vehicle. Where you could run into problems is if that additional driver gets in an accident.