You have to add the address of the cell that contains the formula.
If your original formula is in A1, and A1 is what you're looking to turn
red, then select A1 when creating your CF, and your *conditional* formula
should be:
in the conditional formatting window
click add to get new options
you can have up to three
use the <7 days as the first and past due as the second
it will do the first conditional format it meets.
select the entire row and put as condiotn 1 the formula is =$f6<$B$1-7
format as red
condition 2 the formula is =$f6<$B$1 format other yellow
Assuming f6 is the due date and b1 is todays date.