Solving inclined plane in different axis.

I can work out the acceleration of a block that will take along the plane when the axes are the ones along and perpendicular to the inclined plane. But, when I choose a horizontal and vertical axis I end up with the same mistake as this guy in this thread did:

http://www.physicsforums.com/showthread.php?t=238180

In that thread, it is said that the vertical forces don't add up to zero. I don't get it and hence I cannot solve the problem when the axis is horizontal and vertical. Can anyone help?