I've just purchased the 757 Base Pack and freighter add-on.

Now I have searched the KB for possible answers to my questions but to no avail.

The problem I'm facing is that I am unable to start the engines. Even following the start procedure (according to the Flight Manual) manually and also CTRL+E, this is really getting me frustrated >:(

I have noticed though that the "FUEL CONFIG" light remains on. Not sure whether this makes any difference or not. According to the manual, the "FUEL CONFIG" light only comes on in the following scenarios:
"Center tank pump contains more than 1200lb of usable fuel with the centre pump turned off"....Not that!
"Imbalance in excess of 1800lb between left and right tanks"....Not that!
"Quantity in either main tank is below 2200lb".... Not that!

So after completing the checklist, I go to start the left engine. Turn the switch to "GND" and it sort of starts to spool up but never reaches the mimunum 18% mark on N2 for me to activate the FUEL CONTROL switch. BTW, the APU bleed air is ON.

I would be very happy if I can get this sorted ;)

I noticed that N2 depends on engine type. I don't know how it goes in a real plane, but here it works like this: PW engine N2 reaches about 22-24, RR - 12-14. So if you use RR then turn on the fuel control switch at 12 and engines will start.