i did a little hack to output messages:
add this to your plugin_activation function:
add_option( ‘myplugin_msg’, ‘Error: Something went wrong’);
function myplugin_message() {
if ( $msg = get_option('myplugin_msg') && $_REQUEST['activate'] == 'true' ) {
echo '
<script type="text/javascript">
var el = document.getElementById(\'message\');
el.innerHTML = el.innerHTML + \'<p>'. $msg . '</p>\';
</script>';
delete_option('myplugin_msg');
}
}
add_action( 'wp_print_footer_scripts', 'myplugin_message' );