Design of circuit to reduce flickering of LED
Using a condenser (more often called a capacitor these days) is a good way to filter out flicker (noise) in power coming from a DC generator.  The capacitor would be placed across the two output wires of the generator, (likely after a fuse or other protection device).  Be sure to follow the designated polarity of the capacitor. 

After the capacitor you have your current limiting component (often a resistor) then the LED.

The capacitance value and voltage rating of the capacitor depends on the maximum output of your generator.  The higher the capacitance value the more filtering. A generator powered by a unpredictable power source might give a very wide range of output so be sure to select a capacitor with a high enough voltage ratings to give yourself the best protection from damage.