Depending on you are using or not Maven jobs, there are three different methods for activating Sonar analysis.
Activate Sonar analysis on Maven jobs
If your using a Maven jobs, you just need to add a Post Build set on your project :
Activate Sonar analysis on non-Maven jobs
If you are not using the Maven build process, you can still use this plugin to activate the analysis
Activate Sonar analysis by Ant task
This plugin is useless if you are using the Ant task to define analysis of your projects.
Indeed all the information about the Sonar server will be defined in your Ant build file as explained here