The Nintendo DSi does not have SSH capabilities in it, therefore you can't SSH in.
Sorry to wreck your party, but the uber-h4x0rs at Team Twiizers and HackMii have hacked the DSi by modifying NAND chips, voiding warranties, etc. for a long time.
Sodukuhax was the final DSi exploit found, and that was only possible because of a hole in EA's Soduku.
In fthe future, if I get a Nintendo Developer license, I'll make a free app, with a hole for you lot to exploit.
Or even better, make an app which is free, and lets you run homebrew through it through a secret menu!