Using Alpha Channels
To create a texture with an alpha channel you need to use a tif file. Place the alpha channel in the actual alpha channel of the texture file and save the tif with the alpha channel.
Load it into unity and set it to a materials texture slot with the shader Transparent Diffuse.
You can also use a psd file.
Png files do not seem to work. Try a png with cutout file with solid alpha as the transparent area of the rgb channel.
To just make a given material evenly semi transparent then just change the Alpha color of the Main Color slot in the material.