Not all abilities can be used with all classes - each ability icon will list the classes/Jobs it can be equipped to. As well, each class only unlocks extra 'slots' for abilities for other classes as you level up - you get an extra slot every ten levels or so, with only one slot available at the start. So at level 8 CNJ you'll only have one cross class slot open. Gain some more levels in CNJ and you'll have access to more ability slots.