Can I use stack exchange data explorer to extract data on a previous data dump, say of year 2011 ? Can anyone suggest how I could go about it ?
You cannot. The data explorer only contains the most recent database, and does not have archived former databases. You can download the torrent for a past dump, but I don't know of any way to analyze that dump using the data explorer (you can do it with your own SQL though, I understand that the structure is very similar).