This is what I got from reference document.
If you download the Hibernate Tools distribution you need to place the /plugins and /feature directory into your eclipse directory or eclipse extensions directory. Sometimes Eclipse does not automatically detect new plugins and thus the tools will not be activated.
To ensure eclipse sees these changes run eclipse with the
-clean option. E.g. eclipse -clean.
Rather than downloading and installing, you can do it through update site as well. It doesn't need any additional steps.
1) Go to Eclipse->window->preferences->Install/Update
2) In the pop up window click add and add the following site.
http://download.jboss.org/jbosstools/updates/stable/3)It will show a lot of tools but you will find hibernate tools as a subset of it and tick the hibernate tools check box and click install.