Hello, Moonyasnow
In TROS, in order to get on Frederique's route you need to complete all the other LI routes and then start a new game. When it comes to the first scene in the game with an options menu (in the carriage as you travel through Florentia) a new option should have opened up beneath the others. Selecting this will get you on his route.
I hid his route pretty well as I didn't want anyone stumbling on to it who did not want to romance him.
You can find more information on the route in our guide here: http://blackcross-taylor.com/index.php/guides/2-uncategorised/14-the-rose-of-segunda-walkthrough
Or if you are having any trouble, please do drop me an email here: support@blackcross-taylor.com
Many thanks for playing