Textnow is an application which runs on iOS based devices. You can't use it for Nintendo DSI because it's a different platform with other file-system and files. To send and receive text messages you can use Google Voice (create an account, login and scroll down to bring SMS section in front of you and text to whoever you want) or Yahoo Messenger (create an account, add a friend and when you do be sure to enter his/her phone number into the phone field then when you want to chat chose SMS) by using your browser that comes with your device. Your Nintendo DSI is pretty limited when it comes to calling/texting because it's mainly a handheld console without phone calling and texting abilities.