From the main screen, swipe a message left to right a few times. The message becomes highlighted (color of choosing), which if then tapped the notepad opens. Each individual message's notepad has the ability to create multiple notes. If you choose not to open the notepad after it is highlighted, simply tap the back button, shake the phone, single swipe to either direction, or press the home button.

I find myself taking notes all the time, about the people I message. Things to remember, favors, to do, etc. Having it built in would make life so much easier!