Where is this 'sync label' -option ?
In preferences->plugins->lastfm_pcget, just make sure you have late enough version of plugin.
There are four modes available for sync:
* local to last.fm => if label exists on current track, automatically 'love' track in last.fm
* last.fm to local => if track is 'loved' in last.fm, plugin adds label to track (if it doesn't have it already)
* last.fm AND local => removes tracks label and last.fm-loved UNLESS both exist already
* last.fm OR local => add labels and last.fm-loved IF either one of them exists