I'm not familiar with SMF, but do take a look at the index page code. I'm more than sure it's not on there, but try anyhow.
Most likely, it will be in your styles. If you can add code to your style, it would be there.
On vBulletin, it's in styles, and then the style. Then it will be on the left, it will say footer.
SMF may be similar but I am unsure.
Just looked at post above, he's right. Both right, but he provided a link for ya. This came from my knowledge of using vBulletin and phpBB, so I'm sure SMF is th same, but different in some ways.