hey man, did you make that sig? cuz i saw someone on SGFX wearing it... if you didn't make it, i suggest first asking the person that made it if you can use it, and the crediting him/her... if you did make it, sry 'bout this, just ignore it
try this:
magic wand the white background, right click>select inverse, and then layer via cut... you should now have two layers, one white, and one with the render on it... delete the white render and there ye go.. transparent... hope it works