Far as I know it is the case on ALL turbine engines that fuel should not be introduced until the compressor is spinning at a sufficient rate. The reason being airflow is needed to cool the engine and if insufficient airflow is present prior to ignition you get a hotstart or fire. Only time I can imagine “turning on” fuel prior would be something with FADEC in which case the fuel is still being introduced at the appropriate time by the computer despite you turning it on. The maddog does not have FADEC.
I don’t know who gave you this advice to turn on fuel first 
but these are the normal procedures from Leonardos manual.
In sim what you need to do is engage the start switch using the scroll wheel which will cause to be held down, freeing you to introduce fuel at 20% N2. You can then flip it up when 40% n2 is achieved.