You can't do this from vanilla iTunes. Supposedly the reason is that in a multi-user environment, which iTunes is well suited for, your mom's 1-star reviews of your favourite heavy metal tracks would end up on your iPod and, if you use ratings at all to select songs, mess things up for you. By using the library to track ratings, each user can rate the songs separately.
Sucks for those of us who don't have many people perusing the same music collection.
On OSX, you can supposedly use Rating Writer to mass copy ratings to songs' id3 tags. On Windows, good luck.
You can do a Google search for plugins, maybe you'll get lucky.