Are you managing your menu items through wp-admin > Appearance > Menus? If so, you can add a new "Custom Link". Enter URL: "/wp-login.php?redirect_to=destination_page" Label: "Log In" and click "Add to Menu". That will add a Log In link to your menu, and when you finish logging in, you'd be directed to the destination_page (destination_page is a placeholder for whatever URL you want to enter).
But the new Log In link will not use SimpleModal Login, so you'll need to do one more thing... In the upper-right corner of the Menus admin page, click "Screen Options" and make sure "CSS Classes" is checked. Then, in your menu list, click the disclosure triangle (down arrow) next to "Log In". For "CSS Classes", enter "simplemodal-login".
If you want to be able to return the user back to the same page rather than a single destiation, you may need to add a function to your theme's functions.php. Follow the directions on this page: http://wpfirstaid.com/tag/wp_nav_menu/ except, instead of adding a link to "Home" you'd be adding a link to "Login" and use wp_loginout($_SERVER['REQUEST_URI'], false) where the instructions have home_url( '/' ) .