You can try a couple of things. To get to the site manager you can
try using the url:
http://servername/_layouts/sitemanager.aspx
another option you may have might be to open the site as a network
place. Add a new network place, when it asks for an internet address,
give it the address to the site where your doc library is located
(eg. http://servername/site ). This should open a folder letting you
see all of the document libraries and lists inside your site. Just
find the folder with the name of the document library you want and
move it where you need to. You may have to create a network place for
the site you plan to move it to as well.