How to import a set of png’s in unity3d?
I take it you mean the pngs as in .PNG images…
To import them into Unity, just drag them into your project panel.
Then to put the images into your actual scene in your game, create a new plane by going to:
GameObject > Create Other > Plane.
Then drag your PNG image from your project view onto your plane in your scene. The image will now be on the plane, and you can position and rotate the plane as you wish!!!
Hope this helped and if not, then comment back! 
-Grady
PNG sequences are usually used for animation. You should look for 3rdparty sprite animation tools in Asset Store like ex2d and 2d toolkit. They can store image sequence in sprite sheets and animate them in 3d or pixel perfect 3d.