This should do the trick:
tell application "iTunes"
set theLocation to location of track 1 of playlist "Test"
set the comment of track 1 of playlist "Test" to POSIX path of theLocation as string
end tell
The as string is the key here, otherwise the types are incompatible.
If you want it as a one liner, you can do:
tell application "iTunes" to copy the POSIX path of (location of track 1 of playlist "Test" as string) to comment of track 1 of playlist "Test"
If you want the old Mac style paths with colons (Macintosh HD:Users:rob:Music) instead of Unix style (/Users/rob/Music), you can remove the POSIX path of parts of either of those.