The canvas topic has most of the info about the plugin.
The .ASJSON expression is all the pixel data (red,green,blue,alpha in the range of 0-255) of all the pixels in a format that can be simply loaded into an array object.
A bit of a discription of it's use:http://www.scirra.com/forum/using-pixel-colors-as-reference-for-level-layouts_topic57020_post355255.html?KW=AsJSON#355255
It's tricky to make the behaviors work with per pixel collisions.
Here I got it working with the 8direction behavior by moving the object back to it's last collision free location and setting the object speed to 0.
For the platform behavior the best method would be to create invisible solid walls only immediately around the player. That gives the behavior something to respond to.
Here is a example that creates a platform if pixels are in range:
I increased the size of the test object to increase the sampling area, but it also slows everything a bit.