How do I set the hb51r so it turns on and off after 6 hours?

Manual for HB51RC
http://www.intermatic.com/%7E/media/files/intermatic/products/instructions/timers/hb61rc%20-%20english.ashx
See timer at Amazon
http://www.amazon.com/Intermatic-HB61RC-Digital-Outdoor-Control/dp/B00004W4X5

Plug timer into outlet
Plug lights into timer
Timer turns on automatically at dusk unless you set override feature shown below

Timing cycle repeats each 24 hours
Use selector button to set timer
Timer must be reset after power outage or after unplugging timer.
Push selector button and choose one of following

F > Lights are OFF
O > Constant ON
1 through 9 > timer turns ON at dusk and turns OFF 1 to 9 hours later
d > timer turns ON at dusk and OFF at Dawn
1. through 9. > timer turns ON immediately and then turns OFF 1 to 9 hours later
Maximum wattage 1000
50 mini-bulb-string of Christmas lights is 25 watts

From these instructions, you should push button until 6 displays. Timer will turn ON at dusk and turn OFF 6 hours later
If timer.
If you want timer to turn ON immediately and turn OFF 6 hours later, push button until 6. shows in display.

If timer does not display or does not operate correctly, then timer is suspect. Return timer for another one.