Hi Con, 

The product should come with clear instructions. Generally speaking there are two different types. The first you just add to your fuel tank, while the spray type you are referring to usually has to be applied directly to the DPF. Sometimes this can be done through the pressure sensor hole ahead of the DPF, but sometimes the DPF will have to be removed.

And I'm afraid it's impossible to diagnose the car's loss of power remotely. It could be the DPF, the turbo or any number of other things. A DPF issue would usually be accompanied by a warning light specifically for it.

If in any doubt, bring the car to a good mechanic.