Are autobrakes supposed to slow you down in the descent or when you have touched down on the runway and are braking?

Eskimo is correct.
Autobrakes are wheelbrakes that are monitored by a system that maintains a specific brake pressure.
It senses deceleration rate and applies brake pressure according to selected rate on the panel. (1,2.3, MAX etc)

The system should be used for all landings if it's available. As the main gear touches down, the              autobrakes are activated causing the nose of the aircraft to pitch down. The front wheels can be eased onto the runway by small elevator input. Someting FS2004 does not depict well at all. Manual braking is usually used after 80kts.

Boeing 737 autobrakes:
Autobrake 1 = 4ft/sec decel rate
2= 5ft/second decel rate
3= 7.2 ft/second decel rate
Max= 14ft/ sec above 80kts
12ft/sec below 80kts
RTO= decel rate not controlled.

RTO, further explained
The mode is armed when speed is greater than 90kts  
Activated (ie spoilers extended) if speed  greater than 90kts and you retard the throttles to idle.  
1-3, and MAX is the strength where MAX is hardest braking action.  
 
The 767, for an example has a wider selection, RTO, DISARM, 1, 2, 3, 4, MAX AUTO and OFF. Max. manual braking is always stronger than max. autobrakes"