You "should" see the result str8 in the camera effects tab/room/preview when you set the DOF options... Use auto-update, and full render... Then increase blur intensity as needed after selecting the target... If you don't see the results in there 1st that's cause for worry, but you may just need to increase the value.

Do some tests w/ varying values to compare results, see if it really is or isn't working...

There are 3 kinds of people in the world. Those that can count, and those that can't..