How to Find the Sum of Array Elements Using Idl\Gdl Program?

The TOTAL function might be what you need. If your structure has a field "field1" and you want to add up those values from the structures in your array "structArray", this should work:

field1Total = Total(structArray.field1)

I can answer for the case of using MRDFITS, which you described in the comments of Dick Jackson's answer,

b=mrdfits('/home/bhuvi/Desktop/data/S60501010021alif4ttagfca‌​l (2).fit',
1,range=[3112,3114]) MRDFITS: Binary table. 1 columns by 3 rows. 

GDL> print,b { 1.61571e-13}{ 1.06133e-13}{ 1.06137e-13}

What I think you are getting is an array of structures. And it looks like each structure has a single field, populated with a single float. To illustrate, I defined an array of those structures, using your values for b, and I arbitrarily named the field "data":

b= [{data:1.61571e-13},{data:1.06133e-13},{data:1.06137e-13}]

I get the same output as you when I print it:

IDL> print, b
{  1.61571e-13}{  1.06133e-13}{  1.06137e-13}

So, I'm pretty sure this is what your data looks like. To check it for yourself, help is your friend.

IDL> help, b
B                     STRUCT   = -> <Anonymous> Array[3]

This tells you that b is an array of structures. You can pass the /structure keyword (/str for short) to get info on makeup of the structures within the array:

IDL> help, b, /str
** Structure <beb6f8>, 1 tags, length=4, data length=4, refs=1:
   DATA            FLOAT       1.61571e-13

This says that the first element of the array b, is a structure with a field called 'data', which points to a float value of 1.61571e-13. Alternately, you could just use help with the individual structures by indexing the array b:

IDL> help, b[0]
** Structure <beb6f8>, 1 tags, length=4, data length=4, refs=2:
   DATA            FLOAT       1.61571e-13
IDL> help, b[1]
** Structure <beb6f8>, 1 tags, length=4, data length=4, refs=2:
   DATA            FLOAT       1.06133e-13
IDL> help, b[2]
** Structure <beb6f8>, 1 tags, length=4, data length=4, refs=2:
   DATA            FLOAT       1.06137e-13

I find arrays of structures to be super useful because you can easily look at an individual structure, or you can easily make an array out of a particular field from all the structures. In other words, to get at your data, simply use the structure.field notation, and you have a vector made of the floats from each of the three structures in the array:

IDL> print, b.data
  1.61571e-13  1.06133e-13  1.06137e-13

Finally, to get the sum, use total() as Dick Jackson suggested:

IDL> print, total(b.data)
  3.73841e-13