Followed the below steps to install Activiti plugin (Activiti Designer) in Eclipse - Luna.
1.Exported the certificate using Firefox browser from https://www.activiti.org/designer/update/
2.Exported it as .cer file (you can choose any option) to my local directory (in my case ~/Downloads)
3.Imported the .cer file to the java key store. Used the below command
4.sudo keytool -import -alias activity-bpmn -keystore /Library/Java/JavaVirtualMachines/jdk1.8.0_65.jdk/Contents/Home/jre/lib/security/cacerts -file *activitiorg.cer
5. Added the trust store information in the eclipse.ini file as below.
7.Restart the eclipse.
8.To install the Eclipse plugin, Go to Help → Install New Software. In the following panel, click on Add button and fill in the following fields:
Name: Activiti BPMN 2.0 designer
9.The plugin will be downloaded and after the restart you should be able to see the Activiti Designer editor.