BEFORE YOU INSTALL ANYTHING MAKE BACKUPS BY COPY AND PASTING EVERY FOLDER THAT YOU ARE ABOUT TO EDIT TO A SAFE PLACE.
DUE TO THE NATURE OF THIS ADD-ON DRAGGING AND DROPPING ALL OF THIS INTO YOUR BF2 WILL BREAK YOUR INSTALL.
Only Drag and drop from the folders marked "Drag and Drop." Study the "Example" folders to see where you need to add code in order to enable the weapon to work in BF2.
The reason you cannot drag + drop and overwrite files within the "Example" folders is that you will most likely remove necessary code for any other add-ons you may have added from default BF2. This is especially true if you are adding this Javelin to any BF2 modifications.
Restore the backup copies you copy + pasted elsewhere back over the files you edited.