You can put it in the sidebar file :
Where you want to add it in your sidebar, insert the following code :
<?php
if (function_exists("nggSlideshowWidget"))
{ nggSlideshowWidget($galleryID,$Width,$Height); }
?>
Then, replace $galleryID by the ID of the gallery you want, $Width by the widht of the gallery area, and $Height …
It must works !
(I’m French so sorry for my bad language)