How/where To Enable Cors(cross Origin Resource Sharing)?

I am not able to access cross domain resources from my javascript file using XMLHttpRequest(). I understand that there are a few similar questions, I went through some of them but

Solution 1:

On which server should the Access-Control-Allow-Origin header be set

The server hosting the content you want to read with XHR. The error message does spell this out, it tells you the URL to a resource that you are requesting and then says that the header is not present on that resource.

Is there any setting/configuration/code change which I need to do in my browser/javascript or HTML code for this to work?

No. The browser handles CORS transparently.

As a general query, is there anything the client can/should do for CORS to work or is it purely a server requirement/configuration?

The client has to support CORS. All modern browsers do.

Is the 'Access-Control-Allow-Origin' mandatory in HTTP responses?

No

If not then what does its absence mean - Does it mean only resources from same domain can be accessed or does it mean all domains are accessible(equivalent to *) ?

If a server doesn't specify Access-Control-Allow-Origin then it doesn't grant permission to any other origin to read its data.

Is there any way a client can force a server to add this header?

No (although a browser extension can intercept the response and add the header, this can be useful for testing purposes).