Hi Shreyash,
Try it in this way,
Go to the Activiti zip that you have downloaded. It will contain the database folder and inside of it, you will find create and inside you will find 3 db script files related to postgres database with names activiti.postgres.create.engine.sql, activiti.postgres.create.history.sql, activiti.postgres.create.identity.sql.
1) Now open your DB and create an empty database and run these scripts in that db. You will have all necessary db objects created.
2) Next go to your tomcat's webapps\activiti-explorer\WEB-INF\classes folder and set the db properties appropriately.
3) Make sure you have the postgresql jar file in your tomcat lib folder.
That's it. Every thing should work smooth now.