What you do, is when you save the TGA file, close it in Photoshop, then open the TGA file, go to Channels, select the Alpha channel. Once that's done, select the white area that you want to be transparent, easily done with that logo through use of the Magic Wand tool, and color it in black. It will look red in the Alpha channel. Then, save it again, and convert it to .paa.
And, just in case you didn't understand any of that, heh, I've done it for you and attached it to this post in both .paa and .tga format. Hope that helps you out.