Hi,
Well Im no guru, just a noob who has been through something simmilar, so this isnt really adressed to me, but I think I can help
:
First of all, you need the 2d action pack, foud here:
https://hutonggames.fogbugz.com/?W1150(or on the Ecosystem).
Download and import it, you will then have 'trigger 2d event' and 'collision 2d event' actions, which you should use for 2d collision/triger events.
So basically from my understanding (sorry if it is too basic), 3d and 2d objects use different physics systems/engines in unity. So if you wanted to use a 2d collider, it would also need a rigidbody 2d not a regular rigidbody.
As a result I dont think a 3d object can collide with 2d objects.
So if your entire project is 2d, as a first step ensure youre using a rigidbody 2d component on the object you are trying to collide with and download the action pack I linked to- then youre all set. (also search for a vector 2 action pack, it may be useful for a 2d project)
If your project is indeed a mix of 2d and 3d then I think the workaround would be to position a 3d collider (and regular rigidbody) on the 2d object that you want your 3d character to collide with and all should work fine.
Anyway I hope this helps. Good luck with it!
(edit: oops had wrong link posted, fixed now)