Hello George,
You should rename your old fx_tchdrt.fx to fx_tchdrt_original.fx (or use the one provided) and replace the original fx_tchdrt.fx with the one from the download. If your plane uses "fx_tchdrt.fx" for its effect, then you should see a difference the next time you load the plane.
The other file "fx_tchdrt_DG.fx" is most likely a custom file. If you add the "_DG" to your aircraft.cfg's effect line, it would use that one. Just ignore that one.
Which amphib are you using? What fx files are listed in its [effects] section? It should be right there with the fx_tchdrt.fx line.
The section should look like this:
[EFFECTS]
wake=fx_wake
water=fx_spray
dirt=fx_tchdrt
concrete=fx_sparks
touchdown=fx_tchdwn_s, 1
The water spray in the above example is the water=fx_spray line. You could replace it with a different "spray" effect file. (if you have one). Or you could find one online that is modified already. I didn't see one at the link either, but I'm sure a cure is out there. If you have a plane that does display a decent spray, see what file it is using. Use that one for your other plane too.