How to add a field for IP address?

There is no need to do that, if you just want to track the IP for your records. You can show the IP address by enabling in the submissions page view. Please check this guide for more information:

How to See IP Addresses of Form Submitters

You might also want to check this widget, which can be useful for your purposes:

http://widgets.jotform.com/widget/address_map_locator