is there some kind of additive i need to put in?

It may not be as easy as putting the additive into your system,it may be worth giving your motor a long run in high revs to get the exhaust really hot to burn off carbon deposits from the particulate filter,the additive you are on about is Eolys and is quite expensive and will need to go onto the relevant code reader to reset your system,if it is no better then it may need a forced regeneration of the filter

cdc
September 2012