The Bubblebag directions say you can use fresh or dried plant material - just be sure to freeze it ahead of time. The cold water extraction method uses ice cubes and water to separate the trichomes from the plant material.

Think of similarly to removing candle wax spills from carpet - freeze it with an ice cube and it separates a lot easier, right?

Since it is all going into ice water anyhow, brittle dried material would be rehydrated, so fresh frozen is best IMHO. Also that way I'm not getting little micro-pieces of herb in the finished product making it darker than if I used fresh frozen.