how to add rating tag

First: only WMP accepts rating tags - iTunes stores them locally in a database on the PC.
Then: Add the column %rating wmp% to your columns

Then you may follow the instructions of the MP3Tag help: either add a number between 0 and 255 or asterisks just like in WMP.

WMP has to reread the tracks to see the effects of the rating.
In iTunes nothing of your work will be displayed.