As an ex-Boy Scout (aka reformed pyromaniac), as others have pointed out, signal fires are neat and swell and keen and all of that - when you read about them in survival manuals or see them in Hollyweird movies.
In reality, under survival conditions, they are damn hard to start if you don't have something like a cigarette lighter (How many of you in here have done the rubbing two sticks together BS? I have. It can, literally, take you hours). Flint and steel can work, if you know what you're doing - Fred and Amelia were city slickers. Using avgas to get the thing going makes sense, until you consider that the radio signals had stopped, which means the engine wasn't running, which may mean there wasn't any gas left. (Regardless, for starting a fire, it's a really good way to either blow yourself up or lose a lot of hair in various places. As Ric would say, "I don't want to talk about it ...")
Let's say Fred still had his lighter and it still worked. Have to gather tinder, kindling, and fuel for the signal fire, and then keep gathering the fuel wood/material to keep the fire going. You're on Niku. It's July. It's like 100 degrees plus in the shade ... how long, realistically, do you think one or two people could keep a signal fire going under those conditions before they gave it up from sheer exhaustion?
And as others have pointed out, just building a signal fire may, or may not, have gotten them any notice unless searchers came over when it was dark or nearly so - again - to make a lot of smoke, you need green wood, leaves, something like that, and that makes white smoke. Or you could use upholstery/rubber/etc. from the airplane for black smoke. But still ... it's Niku. It's July. It's 100 degrees plus in the shade. What little water you have is going fast ...