So, I've made a bit of progress with this and have tried to create a symbolic link in the Terminal using the following command.
ln -s ~/Library/Preferences/com.camerabits.PhotoMechanic/StructuredKeywords_UTF-8.txt /Volumes/MEDIA/Dropbox/Pictures/PM/
This creates the file in my Dropbox, however, when I run the same command on the second computer I get the message "File exists".
I've tried using the -f flag to force the link but this just removes the file!
Suggestions would be greatly appreciated!