Your custom ratebook will be saved as a '.dll ' file in the folder:
C:\Program Files\datatex\tng\rates
It will be available from the drop-down selection under 'Trunk Configuration'.
If you are running TNG at more that one branch and they all use the same custom rates, you will need to add the ratebook to each TNG.
NOTE: You cannot copy the ratebook file(.dll) from one TNG to another.
You will need to edit the ratebook in TNG and copy the text, then create a new custom ratebook at the other branch/TNG and copy the text to that custom ratebook. Please take care when copying the entire content of the ratebook to ensure it will work correct.