3000 is very arbitrary, and it doesn't really mean anything, what matters is the difference between the rating of engines. Perhaps something like this would be better:
Very strong = The top engine and higher (as in, Rybka and her derivatives)
Strong = 100 points below top engine and higher (Only Naum and Stockfish are "strong")
Weak = 200 points below top engine and higher (By this, the top 4 engine is already "weak")
Very weak = 201 points below top engine and lower (Komodo and anything below him would be "very weak")
By this Jonny would indeed be very weak, but I'd find such a scale ridiculous.
One can't have it both ways, if "3001" (262 elo below best) is "strong", then "2950" can't be "very weak", as 50 elo is a very small jump.
If you have installed it as UCI engine in a GUI, and assuming you have a Quad, you should have no difficulty...
(see Engine Parameters, what is your GUI?)
If I double-click on jonny.exe, I get:
option name CPUs type spin default 1 min 1 max 128
Thanks Ernest, I am running the Shredder Gui, I found how to change it in the GUI, I thought I could change the parameters when i installed the engine, but I could not! I think I prefer the Fritz or Chessbase GUI.