if everything is installed correctly then this could be an easy fix:
1)Download the latest releases of FBI.cia and FBI.3dsx
https://github.com/Steveice10/FBI/releases
2)copy the .cia file to the cias folder in your SD card
( create one at the root if it doesn't exist)
& copy the .3dsx file to your 3ds folder
( create one at the root if it doesn't exist)
3) insert the SD into your 3ds and power on.
4) open to your download play and trigger your Rosalina menu
( by default ; it should be L+DOWN+SELECT )
---IF THIS DOES NOT WORK, STOP HERE AND USE THE GUIDE --
5) select " change current app to hb
6) exit download play and reopen
- it should open into the homebrew menu-
7) scroll to and open FBI.3dsx
8) select your SD and find your cias folder.
9) inside should be the fbi.cia file that you placed earlier.
press (A) and choose "install and delete" , after its finished you will have FBI as an app on your home menu.
( always do a nand backup just in case something goes wrong in the future.. )