Instructions
Tap the "Nintendo DSi Shop" icon on the DSi's home screen. Tap the "Nintendo DSi Browser" icon. This is icon is labeled "Web."
Tap the "Free" button, followed by the "Yes" button to download the DSi browser onto the device.
Tap the "DSi Menu" button to return to the home screen. Tap the "Nintendo DSi Browser" icon that now appears on the Nintendo DSi's home screen.
Tap the "Go To Page" option and type "YouTube.com" in the address bar using the onscreen keyboard.
Tap the "Go" button to navigate to the YouTube home page. Now, you can search for any YouTube user using YouTube's search feature, view the user's profile and leave comments just as you would on a computer.