Is your problem trying to find a ".Save()" method for your add-in or locating the My Documents path programmatically? If is the former then you need to post your code, if it is the latter then you can use Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)
Yash: Obviously you're doing some extra work here to stop word from saving in its' native file format. Are you trying to get the text out as a string, rich text, ... ? There is a .Save() method somewhere off of your word document you could call to save the .docx. There have been a number of posts on this forum about saving microsoft word documents so I would suggest you search if that is the case.
Something else you might consider doing:
string text = data.GetData(DataFormats.Text).ToString();