Can't Change Default Particle

I am trying to create a simple particle system that emits a bubble texture. I'm using unity 4. I successfully created a bubble material using the Particles/Additive (soft) shader. Then I created a particle system in the scene. The particle material display shows the default particle and when i try to drag and drop the material, the slot is grayed out. I cant click the button to select it manually either. please help!