You are probably using an outdated version of SQL Developer. In this case you see after a restart an error message in the Extensions-Log similar to the following:
SQL Developer 1.x, 2.x and 3.x are based on the Extension Software Development Kit (ESDK) version 1. SQL Developer 4.x requires the use of ESDK version 2. Unfortunately these versions are not compatible. So we decided to support the newest SQL Developer version only.
Please update to SQL Developer 4.x since this is a prerequisite for this extension.