press escape, then select options - controls - conflicting
look in the list for any "F" or "C" keys (that aren't from Simply Jetpacks, obviously), click them and press escape (to set to "NONE")
if you go into options - controls - all, then scroll down to Simply Jetpacks, both the F and C keybindings should be white now (red would mean there are multiple mods using the same key, causing a conflict)
also, when you use your jetpack, you will see "Charging" on the screen next to "Engine-Hover", this is toggled on/off by pressing your [Crouch] key + F key at the same time.