Hi Rajesh,
have you installed the JDBC driver for Impala that is provided by Cloudera? You can download the driver from the Cloudera homepage. Once downloaded extract the zip file to a directory on your hard disk. The directory should contain a lot of jar files and some pdfs. Once extracted open KNIME and go to File->Preferences. In the preferences dialog go to KNIME->Databases and add the folder by clicking on the Add Directory button. If you now open the Impala Connector node dialog you should see a message that the Cloudera Imapal driver is used once you have applied the new settings. If you need to enter a user name and password make sure to add AuthMech=3 into the Parameters field. For more details about the paramters have a look into the Cloudera-JDBC-Driver-for-Impala-Install-Guide.pdf in the extracted folder.
Bye
Tobias