here are the steps:
(1) open your metatrader with all the indicators/EAs
(2) in the "Navigator" window on the left, right click the indicator/EA that you want to edit
(3) select "Modify"
That will open the metaEditor program that allows you to edit and compile code.
Just remember, you can only edit code if you have the MQ4 file. If you only have the compiled ex4 file then you can't edit it (because it's already compiled into binary).