Yep, you've installed the OpenStudio Legacy plugin, not the OpenStudio Plugin.

The OpenStudio Plugin is installed when you install the full application, and can be found at openstudio.net.

The FAQ meant to have a section called What is the difference between the current version of OpenStudio and the Legacy Version of OpenStudio? but it's pretty empty.

Short story: if you like to work in the IDF Editor or your favorite text editor and are only looking for a plugin to modify geometry, choose the Openstudio Legacy plugin. If you want a full suite of applications using a GUI, choose the full OpenStudio.

A bit of detail:

• The "Legacy" plugin as the name implies is an older version that was kept available for a number of reasons while the active development moved to the full OpenStudio suite. The legacy plugin is no longer under active development. The legacy plugin is initially meant for geometry creation and tweaking. You can import a full IDF, modify the geometry, and save again. (That's probably the main reason why it was kept alive, because during the reverse translation from the IDF to a full version OpenStudio Model (OSM), a lot of information is lost). The Legacy plugin also has a couple of neat options for visualizing some variables directly on the geometry.

• The Full OpenStudio application includes a sketchup Plug-In for geometry creation, OpenStudio Application which is a GUI for EnergyPlus, the Parametric Analysis Tool (PAT) for evaluation design alternatives, and the ResultsViewer to visualize the results, direct access from OS App and PAT to the BCL, a shared collection of measures, etc (see here). Last but not least (I'd personally call it preponderant), the openstudio ecosystem is built atop an object-oriented model and a SDK, and as a result there's an API in several languages (ruby is the default but you could use java, python, c#, etc) that allows you to heavily leverage scripting to automate tasks, evaluate design alternatives, etc.