Please be aware that you have to do this at your own risk. Adding Testing Feed mean that you aware and ready to download beta updates, if available, of any applications in Preware. You must see here for complete explanations.

In this blogpost, I only write down for my Palm Pre. Here's how to do it:
1. Open up Preware, let it finish loading applications
2. Swipe down top left corner of the screen, then choose "Manage Feeds"
3. Scroll to the bottom until you see New Feed Box
4. Fill in the box, make sure the URL doesn't have a slash at the end:

a. Name: webos-testing-all
    URL: http://ipkg.preware.org/feeds/webos-internals/testing/all 

b. Name: webos-testing-arm7
    URL: http://ipkg.preware.org/feeds/webos-internals/testing/arm7

5. Make sure the IS COMPRESSED option is set to YES
6. Tap "Add Feed" and tap "YES" for popup alert
7. Gesture swipe back, and Preware will request you to reboot the device.