OSM Buildings is an additional layer to existing web maps. It is currently works with LeafletJS and OpenLayers, so have one of these running in your website.

I assume, Leaflet map engine should be already integrated in your html page.
Make sure you are running at least version 0.7

Download latest version of OSM Buildings, including sample data, compatible with at least Leaflet 0.7
The download contains source code, data samples and a build system. The library files are located in the dist folder.

Alternatively head over to GitHub directly and pull the source.
Make sure you read the Readme and Changelog.