Pentaho spoon error: "Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections."

I'm new in Pentaho environment. I installed pentaho CE recently on another computer and couldn't preview a simple mongoDB input step result which fetches data from mongodb. It is giving me the following error message:

Blockquote org.pentaho.di.core.exception.KettleTransException: There was an error calculating the change data capture date range, it probably involved log table trans_logs.

Error occurred while trying to connect to the database Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.

at org.pentaho.di.trans.Trans.calculateBatchIdAndDateRange(Trans.java:2238)
at org.pentaho.di.trans.Trans.prepareExecution(Trans.java:985)
at org.pentaho.di.ui.spoon.trans.TransGraph.debug(TransGraph.java:3958)
at org.pentaho.di.ui.spoon.delegates.SpoonTransformationDelegate.executeTransformation(SpoonTransformationDelegate.java:893)
at org.pentaho.di.ui.spoon.Spoon$29$1.run(Spoon.java:8503)
at org.eclipse.swt.widgets.RunnableLock.run(Unknown Source)
at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Unknown Source)
at org.eclipse.swt.widgets.Display.runAsyncMessages(Unknown Source)
at org.eclipse.swt.widgets.Display.readAndDispatch(Unknown Source)
at org.pentaho.di.ui.spoon.Spoon.readAndDispatch(Spoon.java:1366)
at org.pentaho.di.ui.spoon.Spoon.waitForDispose(Spoon.java:8022)
at org.pentaho.di.ui.spoon.Spoon.start(Spoon.java:9277)
at org.pentaho.di.ui.spoon.Spoon.main(Spoon.java:692)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.pentaho.commons.launcher.Launcher.main(Launcher.java:92)

Caused by: org.pentaho.di.core.exception.KettleDatabaseException: Error occurred while trying to connect to the database Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.

at org.pentaho.di.core.database.Database.normalConnect(Database.java:472)
at org.pentaho.di.core.database.Database.connect(Database.java:370)
at org.pentaho.di.core.database.Database.connect(Database.java:341)
at org.pentaho.di.core.database.Database.connect(Database.java:331)
at org.pentaho.di.trans.Trans.calculateBatchIdAndDateRange(Trans.java:2061)
... 17 more
Caused by: org.postgresql.util.PSQLException: Connection refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.
    at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:215)
    at org.postgresql.core.ConnectionFactory.openConnection(ConnectionFactory.java:64)
    at org.postgresql.jdbc2.AbstractJdbc2Connection.<init>(AbstractJdbc2Connection.java:144)
    at org.postgresql.jdbc3.AbstractJdbc3Connection.<init>(AbstractJdbc3Connection.java:29)
    at org.postgresql.jdbc3g.AbstractJdbc3gConnection.<init>(AbstractJdbc3gConnection.java:21)
    at org.postgresql.jdbc4.AbstractJdbc4Connection.<init>(AbstractJdbc4Connection.java:31)
    at org.postgresql.jdbc4.Jdbc4Connection.<init>(Jdbc4Connection.java:24)
    at org.postgresql.Driver.makeConnection(Driver.java:410)
    at org.postgresql.Driver.connect(Driver.java:280)
    at java.sql.DriverManager.getConnection(DriverManager.java:664)
    at java.sql.DriverManager.getConnection(DriverManager.java:247)
    at org.osjava.sj.loader.SJDataSource.getConnection(SJDataSource.java:101)
    at org.osjava.sj.loader.SJDataSource.getConnection(SJDataSource.java:81)
    at org.pentaho.di.core.database.Database.normalConnect(Database.java:431)
    ... 21 more
Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:589)
    at org.postgresql.core.PGStream.<init>(PGStream.java:61)
    at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:109)
    ... 34 more

The problem seems there is an error connecting to Postgres db. If I'm not mistaken. Do I've to install Postgre separately? If so, how can I install and configure? I'm stacked here. Would anyone help me out?

Yes, you have to install and configure postgresql.