function_exists should be used after the function name in a theme not before.
add_action( 'loop_start', 'add_slider' );
function add_slider() {
if ( function_exists( 'soliloquy' ) ) {
soliloquy( 'slider', 'slug' );
}
}
This checks to make sure the plugin is active before outputting a function and/or markup otherwise you might get a error like call to undefined function.
