Hello.

The gauge itself has two wires plugged on to the two studs sticking out of the back of it. One wire is green with a black stripe that goes on the driver's side stud and the other is yellow and goes to, you guessed it

, the passenger side. At the front of the gas tank, where the fuel line goes in, there is a stud stiching out of the front of the sending unit. There will be the other end of that yellow wire hanging around back there somewhere. It should be pushed onto that stud. However, usually when people tell you that the gas gauge isn't hooked up, they are lying. It is hooked up but just isn't working, usually due to the fact that the float on the sending unit is no longer able to float because of some holes in it. If it is, in fact, not hooked up, that would raise the interesting question of why was it unhooked in the first place. Think 'shorts', fire hazard, etc... Hope that helps.
