Create a new rule, and select "Use a formula to determine which cells to format."
In the formula bar, type =A1="x"
Click, OK, and then in "Applies to," type =$D$1:$D$1000
Just make "1000" into whatever number you think will cover all of the data that you'll add.
If you just want it to fill cell D when there's any text in A1, change the formula to =A1<>""