Possibly the files do contain the Rating tag. Perhaps you have not enabled tag writing in the field specific options or the file format cannot contain Media Center tags.
You can make a library backup in MC11 and restore the backup zip file in MC12. Then the MC12 libary will have everything you had in the MC11 library.
File > Library > Backup/Restore ...
Edit
In your other post you mentioned that some of your ratings have disappeared after using MC12 for some time.
Do you have the "Update for external changes" option enabled in the Auto-Import Options? When this options is enabled and MC detects that a file has changed it tries to reread the file info and depending on the file format and available physical tags this may change some of the library field values.
