GdkWindow *gdkWindow;       /* the GDK window wrapped around your X window */
GtkIMContext *im_context;   /* the context for the keyboard input method */

/* create and map your X and GDK windows */
...
gdkWindow = ...;            /* see snippet 3 on how this is done */

/*
 * use this to show the keyboard
 */
im_context = gtk_im_multicontext_new();
gtk_im_context_set_client_window(im_context, gdkWindow);
hildon_gtk_im_context_show(im_context);

while (gtk_events_pending())
    gtk_main_iteration();

...

/*
 * use this to hide the keyboard
 */
hildon_gtk_im_context_hide(im_context);
gtk_im_context_reset(im_context);
g_object_unref(im_context);

while (gtk_events_pending())
    gtk_main_iteration();

...