Ok I managed !! 
Getting inspired by this post
This is a problem with recent PMS versions => would be so nice the staff fix that and notify in this thread because I am sure lot of people spend lot of time trying to find a way !
So I uninstall PMS from the Synology
I downloaded that previous version of PMS in order to downgrade: 1.17.0.1841-d42cfa161
And installed :
Then I did the itunes playlits import again … all the ratings appeared … hallelujah !
To be noticed in the XML file aswell I deleted the localhost mentions so my paths are constructed this way : file:///volume1/
