I have a MTD Lawntractor 18hp and have tried to start in correct manner. Sat on seat, dis-engaged cutter bar, pushed brake pedal down and handbrake in operation. Battery in good condition - fully charged.

Any ideas why nothing happens - do I replace fuse and where is it?

Spray some starter fluid on the air filter for about 2 seconds and only use gas that's ethonal free 100% GAS

No, engine doesn't even turn - could be fuse ?

It could be the fuse. You can follow the positive battery cable. It will take you to the solenoid. The fuse will be near it. Try pulling back on the deck handle while you try to start. The deck switch being out of adjustment is the most common problem I see.