[eluser]ecsyle31[/eluser]
The api is the SSI. You won't install SMF inside codeigniter at all, you will install it separately and just include SSI.php in CI app.
You could so something like this:
/path/to/webroot/forums/SMF_FILES_HERE
/path/to/webroot/system/
/path/to/webroot/index.php
then in your codeigniter app, include SSI.php - <?php require_once '/path/to/webroot/forums/SSI.php'; ?>
Now you have access to all the methods available in that file, as well as the $context array for the currently logged in user.