Program to find whether addition of two numbers in an integer array returns the value 'k' entered by the user .

You don't need to define values for TRUE and FALSE. You have tagged this question as being for C++ and the language has the keywords true and false defined in it and that is all you need.

Here is some pseudo code that should give you an idea.

while( looping )
{
   if( array[m] + array[n] == valueK )
       return true;
}
return false;

The looping part should look like the loops for a bubble sort and check every possible pair of values and not the same one twice. Here is a "dumb, brute force" way to do that. I will leave it to you to figure out a more optimal way.

// dumb loops - not recommended but the program can work with them.
int last = size - 1;
for( int m = 0; m < last; ++m )
   for( int n = 0; n < last; ++n )
   {
      if( n == m )
          continue;    // same index - skip this one
      // evaluate sum of values here
   }

If find that you can't figure out a better way to write the loops then look up some code on implementing a bubble sort. That should show you an example of better loops.

int last = n - 1;
    for( int i = 0; i < last - 1; ++i )
        for( int j = i + 1; j < last; ++j )
        {
        }