How do you get nel to go to other areas? she only goes into the slime forest and theres no dialogue option to ask her
Try increasing friendship levels with her. I believe Deimus already answered the question down below. She will pick any one of the following locations from random according to your friendship level with her.
Level 1 - Slimes
Level 2 - Hornet
Level 3 - Jellyfish
Level 4 - Rockspitters
Level 5 - Fire Rays
Level 6 - Dark Fairies, I think. I didn't progress that far yet.
You cannot ask her to go to a specific location yet. (as much as I would want that feature to be implemented, since I end up quickloading the game more times than actually having to autofight because she chose someplace I didn't want her to go )