For adding the new instance in NAV server follow below steps.
a. Open the Microsoft Dynamics NAV administration server
b. Click on Add instance
c. It will open new window as show in figure.
d. Now fill the all the detail as show in below image.
i. Username: your computer login username
ii. Domain Name: if you have domain server name then write this otherwise you have to write your Computer Name.
iii. Password, Confirm Password: Your computer login Password.
e. Now close the window by click on Ok button. It will add new instance to NAV server its show like below. Now open this instance detail and go to edit mode and change server instance name, database name, server name etc. and then save it .
f. Now right click on instance name and start the service. As show in below image.