You aren't using enough chlorine to kill the algae. You have to get your ppm up to 20 and keep it there until the algae is dead. Once the algae is dead you will be able to hold free chlorine. Here is a bleach Calc written by one of our fourm members that can help you see how much to add:

http://www.hal-pc.org/~mwsmith2/BleachCalc262.exe

You have to be patient and dedicated - you will get your pool cleaned up!