Jeffrey Friedl has a details post on installing Lightroom Export Plugins.
“A plugin consists of a folder worth of files, with the folder having a name that ends with “.lrplugin” or “.lrdevpluginin”. Installing a plugin involves simply moving the plugin folder to a place where Lightroom will find it, then restarting Lightroom.
The SDK zip’s “Sample Plugins” folder contains a few example plugins, such as flickr.lrdevplugin. Other plugins offered for download on the web will likely be offered as individual zip files. Upon downloading, you’ll want to unzip to create the plugin folder, then move that folder to the proper spot as described below.“
Read the entire post.
Even if you are not a developer, you should consider downloading the SDK. Why? Because, while rudimentary (these are example plugins), the Flickr and FTP plugins are very useful. I can see the FTP preset replacing my current Transmit Droplet system. I may even look into the code to customise them (I currently don’t have a clue about Lua programming, but I’m sure it’s just a matter of the learning curve!