I too have had this problem, so I wrote up a quick applescript. It requires the EXIFTool to be installed from here: http://www.sno.phy.queensu.ca/~phil/exiftool/
Simply paste this script into Script Editor, save it to file and copy it to your Library/iTunes/Scripts folder.
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tell application "iTunes"
-- act on particular tracks or all tracks currently displayed
if selection is not {} then
set sel to selection
else
set sel to file tracks of view of front window
end if
-- loop through each selected track
repeat with aTrack in sel
set loc to aTrack's location
set prettyloc to (quoted form of POSIX path of loc) as string
set popm to do shell script ("exiftool -Popularimeter " & prettyloc & "|sed s'/^.*Rating=//g;s/ .*//g;'")
try
set trackrating to round (popm / 2.56) as number
end try
if class of trackrating is integer and trackrating > 0 then
tell aTrack
set rating to trackrating
end tell
end if
end repeat
end tell
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