iTunes doesn't store its ratings in file tags.
So MC can't import them from the files.
To bring ratings from iTunes to MC, i do the following "trick":
The field composer is one, that both programs have in common.
If you don't use it otherwise you can sort on ratings in iTunes, select the records with rating "1 star", go to Information (cmd-I), and fill the field composer with "1 star" and so on.
On the MC side you can retrieve the composer field, sort your records accordingly and then tag your ratings in MC.
This is a one time "trick". I would not want to do it every time i sync MC with iTunes.
I believe JRiver will provide an easier way in the future.