I recently generated a pet, attuned it to an element but still can't add it to my party. I already purchased the 2nd slot from the Hunter. I still don't see the + sign.

pets don't need to be adult to be added to the party- if that was the case, everybody would need to wait for a few weeks while their starter got old enough to add.

so to make sure
> you're going to the party page
> you're going down to the paired dropdown menus and using the second one to select the pet you want
> you're waiting from the pet preview to swap from displaying Spirit's info to his
> you? apparently cannot see the + sign that is located next to those dropdowns? that shouldn't actually be possible, I don't think (even when you can't actually put a pet on your party it's still there and usable- you just get a popup saying the pet can't be added, if I remember right. And the only thing that I can think of that actually stops a pet being put on the party is it being too high of a level, and that isn't going to be the case with a newly generated pet)

nope, starter pets are basically just regular old mystical generated pets but with a 'starter' tag instead, with all that entails- including starting life at the start of adolescence.