Vijay,
XML is extensible markup language. It looks very similar to HTML (if you "view source" on a webpage, you can see some html coding). Basically, XML is a user-defined structure for storing data in such a way that it can be searched and retrieved very easily.
In Excel 2007, Microsoft started using this "behind the scenes" method for storing Excel spreadsheets (and other Office documents). That's what the "x" on the end of "xlsx" or "docx" means. "Hey, I'm in XML format." This allows 2010 to open 2007 documents (and vice-versa) without special adapters or code.
So, you really can't do XML work with Excel. I suggest you use DreamWeaver (Adobe) or similar editors (even wordpad would work) if you need to develop some of your own xml data.
This is from my knowledge of XML (yes, I've used it), and Excel's Help files, and my own fast research. I stand to be corrected by one of the gurus here.
Don