Leveling the bed with two screws:
Frontally: http://youtu.be/3MagXojsxTo
Laterally: http://youtu.be/LGQ_Vna2ZAw
Why are you actually leveling the bed? What you need is for it to be in tram with the print head. i.e. the head needs to move in a parallel plane to the bed. “Leveling” is the wrong word for this procedure, as it can be at any angle, as long as it’s parallel to the print head’s movement.
While I agree “level” is the wrong concept, (leading to the mistake made here) tram is also the wrong term. Tramming refers to making the tool (usually a mill’s spindle) perpendicular to the work surface. A printer’s build platform needs to be squared to the axes. While we’re at it, it’s also good to have the axes square to each other.</Pedantry war>
@Dale_Dunn , given that the work piece on a CNC mill sits on the bed, tram is most definitely the correct term. CNC mills have the same adjustments, though usually located elsewhere. We are making the tool perpendicular to the work surface.
Yes @Carlton_Dodd , I level the platform because i previously level the X-axis, Do not demand much my English 
@Manuel_Palacios , using the bubble level on the axis and the table is a good rough measurement, but you will soon find that the corners of the bed are still at different Z positions. Bring the extruder down to one of the corners, just close enough that a piece of paper barely slides between the extruder and the bed. Then check at the other 3 corners and adjust them until they all have the same drag on the paper.
There are several ways to level the platform, when I want to do it accurately I connect a multimeter pole to hot-end and the other to a washer on the platform. I do a similar test to the piece of paper but testing continuity. I do it only in three points.
If i want to level the printer quickly, first i level it frontally like in the first video and later i do it laterally like in the second. @Dale_Dunn
@Manuel_Palacios , I like the multimeter idea. You could use that with feeler gauges to set the first layer height too, as long as there’s no plastic ooze on the nozzle.