You can specify the location of AtomicParsley within the script:
on run {input, parameters}
set AtomicParsleyPath to "/Users/G/Downloads/AtomicParsley-MacOSX-0.9.0/AtomicParsley"
tell application "iTunes" to set selectedTracks to location of selection
repeat with selectedTrack in selectedTracks
tell application "iTunes" to set trackPath to POSIX path of selectedTrack
# display dialog trackPath
set theCommand to quoted form of AtomicParsleyPath & space & quoted form of trackPath & " --advisory explicit --overWrite"
# display dialog(the_command)
do shell script theCommand
end repeat
end run