XBMC advancedsettings.xml

<advancedsettings>
    <videodatabase>
        <type>mysql</type>
        <host>192.168.100.120</host>
        <port>3306</port>
        <user>xbmc</user>
        <pass>xbmc</pass>
        <name>xbmc_video</name>
    </videodatabase>

    <musicdatabase>
        <type>mysql</type>
        <host>192.168.100.120</host>
        <port>3306</port>
        <user>xbmc</user>
        <pass>xbmc</pass>
        <name>xbmc_music</name>
    </musicdatabase>

    <pathsubstitution>
        <substitute>
            <from>special://masterprofile/Thumbnails</from>
            <to>nfs://192.168.100.120:2049/Volumes/External/Downloads/Thumbnails</to>
        </substitute>
    </pathsubstitution>
</advancedsettings>

The ATV2 has the pathsubstition part for shared thumbnails. The Mac Mini doesn't, but the symlink was pointed to /Volumes/External/Downloads/Thumbnails