Reach the storyline of Tenkaichi Boudouki. When choosing your four fighters in Tenkaichi Boudouki,
1) choose Son Goku in the first grid from the very left,
2) leave the second grid from the left blank,
3) choose Piccolo in the third grid from the left, and
4) choose Kurirun in the remaining grid (ie, the fourth from the left)
And then the hidden character Chi Chi will show up and fight with Son Goku.