To register Decompose, simply enter your registration name and number exactly as in the email you received when asked at the application startup. The easiest way is to simply copy and paste them from the email you received to the registration window. If you are entering the information manually, make sure you enter both the registration name and number exactly as in the email since the information is case sensitive.
This is the limitation of the demonstration mode of Decompose. Simply purchase a license and register Decompose.
Decompose requires that you identify the pixels of the background, foreground and contour in order to extract the desired elements from the image. Initially all pixels are flagged as background pixels. Generally, you will switch to the Contour color and use the Brush tool to draw the contour of the object to extract. Then, you will switch to the Foreground color and use the Fill tool to fill the object to extract. If you need more information make sure you read the Decompose tutorial.
To uninstall Decompose simply move the application icon to the Trash.