You could use an in-line if statement

=iif(Datediff("d",Fields!ProcessStartDate.Value,Today()) <=7, "Less than 1 week", iif(Datediff("d",Fields!ProcessStartDate.Value,Today()) <= 14, "1 to 2 weeks", "More than 2 weeks"))

I've editted your statement slight, as your logic stated value for less than 7 days and, more than 7 days and less than or equal to 14 only. It did not include logic for 7 days, or more than 14. You can edit as you see fit.

Edit: For some reason < is being converted to &lt; in the code box. You'll have to change these yourself.