sum filtered rows/columns

Hi
I want to be able to sum a column of data after a filter has been applied using the plug in column filter.

I found this thread: http://datatables.net/forums/discussion/2053/fnfootercallback-sum-column-after-filter/p1

Which partly relates to the solution but I'm unclear on how I can use the object created:

._('td:nth-child(4)', {"filter": "applied"})

I'm a novice at jquery - I assume I need to go through each cell in the column and add the value but I havent a clue on how to start this

Can you point me in the right direction please

Thanks