It’s a simple plugin to install, upload the unzipped plugin to your WordPress plugins folder, activate and in your WordPress admin area go to Settings -> LinkedIn Button. There you can choose where to have the button appear automatically, and where on the page or post it should appear.
If you don’t want the button to appear automatically, just uncheck all the Automatic Button Display options and you can add the button manually to a post or page using the settings on the edit page.
The plugin comes with three button variations, but you can create your own button and styles for the plugin if you like, check out the FAQ on the WordPress.org site.
You might also like to read my blog post on sharing on social networks and have a think about what sites your target audience are most likely to share.
While FaceBook and Twitter are both very popular for sharing, LinkedIn has a more focused network than Twitter, and often a more professional slant than FaceBook. If you’re blog is in any way business focussed this plugin might be a good way to increase exposure of your content – let me know if you find it successful for you!