Setting up controlled vocabularies
To enable the semantic editor you first have to set the controlled vocabularies you want to annotate with the semantic editor.
You need to install the Vocabulary Control module (on GitHub), which depends on the Accesspoint module (on GitHub). If you want to statically import external vocabularies, you will also need to enable the Accesspoint Store module (it's shipped as submodule of the Accesspoint module). In order to actually use vocabularies you have to define ontology paths and groups with the Pathbuilder module first as described in the Installation Guide.
The Vocabularies Guide explains all necessary steps to configure accesspoints and vocabularies and how to use the vocabularies.