Okay if you hear the fuel pump then it isn't the problem.
Do you smell gas after trying to start it a few times?
The immobilizer is essentially the entire anti-theft alarm system in your car. The Smart Key(the key fob) contains a chip inside that communicates with the car allowing the car to start normally. If the key is not recognized the car will not start.
If you crank the car and release the key after 3-4 seconds, what lights are on the dashboard? Like is the Check Engine Light on? or the Battery Icon, etc.
If you are not very mechanically inclined, hop into your car, turn the key and let it crank for a 3-4 seconds and release the key(DO NOT TURN THE KEY TO THE OFF POSITION). There should be symbol lights on your dashboard. Take a photo of the gauge cluster with either a digital camera or a smartphone and upload the photo onto here. I will look at it and try and help you further.
'08 ML550 - P2, Int App Pack, Heat Pack, Distronic, Rear Seat Ent, Keyless-Go,
Voice Control, TH, CD, and iPod.
'10 S550 - P2, AMG Pack, Rear Side Shades.
'03 C240 - C2, CD, Tele Aid
'08 GL450 - P2, Lthr Pack, Rear Seat Ent, iPod, Voice Control, TH, Keyless-Go, Run Brds.
'06 ML500 - Prem Pack, Comf Pack, Light Pack, Ent Pack, Sunroof, Airmatic, Parktronic, Rear Airbags, TH, 3-Zone, Keyless-Go, Nav, Pwr Liftgate.
'98 ML430 - M4, Sunroof and Bose.