Anywhere in the title or main menu system of the game should be fine.
So for example, it'd be ok to have a title menu like:
Continue
New Game
Options
And then put Credits under the options sub menu like this:
Sound Options
Video Options
Input Options
Credits
That would be fine.
This is just an example, however you want to lay things out is fine, just so long as the credits are easy to find from the title screen you should be fine.