Creating and saving brushes
To create a brush, open the brushes engine by pressing Window > Brushes in the main menu. A brushes engine window will appear with lots of settings for a selected brush. Make sure you have a brush tool from the tools palette selected, because the brushes engine will not be active with some other tools. Creating a brush is simply a matter of copying the settings that will be posted below. Once the settings are defined, you can save the brush by pressing the new brush button in the bottom of the brushes window.
Clouds brush # 1
Clouds brush # 2
Clouds brush # 3
Testing
When you save the brushes, you can later pick them from the brushes list. To draw the clouds, pick the brush and draw with a 100% opaque white color. Here's a sample result: