There is a subtle difference between these two, but both probably do what you need:
Code:
=SUMIFS(F2:F29, C2:C29,"PSL", D2:D29, "=")
or
=SUMIFS(F2:F29, C2:C29,"PSL", D2:D29, "")
The first formula will only sum when the cells in column D are completely empty. The second formula will only sum when the cells in column D are completely empty or contain a zero length string.