Note: Confirmed that according to the TPP 5.0 release notes that it was removed due to the default behavior being changed to not have the plotting turned on in ProteinProphet:

http://tools.proteomecenter.org/wiki/index.php?title=TPP:5.0.0_Release_Notes

"Stop plotting sens/err png by default; removed NOPLOT option. To produce png, use new PLOTPNG option."