GCFScape is the program to use.
Make sure your .NET framework is updated (Nem's site has the proper update.)
env_explosion seems to be buggy for some reason, at least for me. Each time I've used it in a map it seems to crash when activated.
Via This Output is what the trigger should do to the entity. Should it enable the sprite, if it's a sprite entity you're targeting? Should it play the sound if it's an ambient_generic?
2) They are invisible cause you accidently click the circle in the middle of the props. If you make it bigger it changes the visability of it. So delete it and place a new one and I promise you it will work.
Actually, you don't have to delete it, you just have to reset the fade distances (MinFadeDist to -1 and MaxFadeDist to 0).