If you read the manual (I didn't read it either!) it tells you that you can only use the detergent attachment in "Low Pressure" mode which is when you remove the high pressure nozzle.
Basically if you're cleaning the driveway etc. then just spill the detergent onto the ground and wash it away or don't bother using it.
I use it for washing the car alright and then use the high pressure nozzle to wash off the suds afterwards.