You need to make choices as to which abilities you wish to use. You only have access to one skill per 5 levels of your current class, so up to 10 at level 50. As a level 8 conjurer, you would only have access to a single cross class skill.

Also note that once you equip a job cristal (transforming an archer into bard, for eaxample), you gain new abilities and some hefty stat bonuses, but you also lose half of your cross class slots (from 10 to 5 at level 50) and you can only pick them from the two classes most relevant to yours.

If you are having problems actually slotting the abilities to your bar, then you need to first check them in the cross class abilities section. Once checked, you'll be able to drag them anywhere you wish.