Transperancy?

When i try to make my objects transperant in the Materials tab (shaders and the slider under the colors) it wont get transperant. I’ve tried to make it totally transperant (invisible) but its still as solid as a rock. What do I do wrong?

EDIT: I havent got it working on version 2.40 and 2.41

Select the mesh that you want transparent, in the materials buttons (F5Key) turn down the alpha transparency slider which is on the materials tab, and then on the Mirror Transp tab click on either ZTransp or RayTransp. That’s it.