Hi Nalini,
mojoPortal comes with support for Google Custom Search so all you need to do is input your Google Custom Search ID in the API Keys section of the Site Settings. To get there go to Administration > Site Settings > API Keys. If you want the google search to be used by default, select it from the Default Site Search Engine box.
Your ID should look something like this: 017268123402775890745:jfnmabcaxzs
HTH,
Joe Davis