I think you can have only one FTB-modpack one your server at the time. So if you want to install the Trident FTB-Modpack you have to choose "Minecraft (Curse Launcher / Feed the Beast)" in the gameswitching tab an click "reinstall". There should be a drop down menu where you can choose the Trident-Modpack. Keep in mind, if you click reinstall and choose the new modpack the old ftb modpack which you had installed will be deleted.
I hope I could help you,