I just installed asterisk & asterisk GUI. 

Then I have also defined the username and password in my manager.conf. 

So,I went to Admin > Telephony Server Settings and with the credentials provided I have successfully enabled this integration.

My next question is how do I receive or make calls like what one of your documentations show at bottom?

https://www.manageengine.com/products/support-center/help/adminguide/configurations/helpdesk/computer-telephony-integration.html#telephony

Can someone please assist on this?

Thanks in advance