Alamo has two locations on site, one at the Dolphin Hotel and one at the Car Care Center. The one at the CCC will pick you up at your resort, the one at the Dophin does not.
I would make the reservation before arriving at WDW, and call Alamo the day before to schedule a pickup time for your rental the number is (407) 824-3470.
If you choose to use the Dolphin location you can taxi there from POP for about $5-$7
The hours for the CCC office are M-F 6am to 8pm, and Sat and Sun from 6am to 6pm.