Remove the engine cover to the right of the oil cap (the smaller one, it's held in place by two 10mm bolts). Uderneath you'll find the fuel rail (the injectors are attachd to it). Follow the fuel rail and you'll hit the fuel hose.

 

Not trying to be a jerk, but if you can't figure out how fuel is getting to the engine then you probably shouldn't be messing with this. Honestly, if you install that thing and something goes wrong the service dept is going to have a field day with you. Personally, there is no way in hell-o I would install that crap on a BMW engine.

If you want to improve fuel economy buy a good bottle of valvoline brand fuel injector and valve cleaner. Better yet replace your fuel pump, clean or replace your air filter and replace the plugs. That will do more for the life of the engine and improve econ as well.

 

The second part is spot on but installing the magnet is not a big deal. You simply attach two magnets to the fuel hose with zip ties. It doesn't do anything and it can't screw anything up unless it falls off and gets into moving parts.

Personally I would never install one of those things, but if you do decide to install it make sure it won't fall off and land where it shouldn't.