How to add ADDTHIS

I'm new to this and need help on adding the addthis to my gallery so that my users can share on social networking sites. I have no idea where it need to go in the code in the theme.

Gallery version = 2.3.1 core 1.3.0.1
API = Core 7.54, Module 3.9, Theme 2.6, Embed 1.5
PHP version = 5.2.11 cgi
Webserver = Apache
Database = mysqlt 5.0.89-community, lock.system=flock
Toolkits = ArchiveUpload, ImageMagick, LinkItemToolkit, Thumbnail
Acceleration = none, none
Operating system = Linux cl53.justhost.com 2.6.31.9-grsec #1 SMP Thu Dec 24 05:30:59 CST 2009 x86_64
Default theme = matrix
gettext = enabled
Locale = en_US
Browser = Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/4.0.249.89 Safari/532.5
Rows in GalleryAccessMap table = 62
Rows in GalleryAccessSubscriberMap table = 4666
Rows in GalleryUser table = 4
Rows in GalleryItem table = 4656
Rows in GalleryAlbumItem table = 49
Rows in GalleryCacheMap table = 0

Hi, I'm new to this as well. I extracted the addThis folder and put it in the modules folder, but it doesn't show up when I go into the admin section. I went to the get more plug-ins section and it gave me a list of available plug-ins to download, but this one wasn't listed. Just curious if there's a way to find any new plug-ins I've uploaded to the folder or if there's another way to go about it?

Thanks!