How to allocate memory using malloc in mplab x ide, XC16 compiler?

Using malloc etc. is the same as in any C implementation. By default, however, the linker in XC16 will not allocate a heap (from which memory for malloc is taken). You will need to tell the linker to allocate a heap in Project Properties|xc16-lc|General|Heap size. The size of the heap must be larger than the largest memory allocations you are making, plus some overhead (see XC16 documentation for details).

Keep in mind that a lot of malloc/ralloc activity may fragment the heap, and your heap is not large. So, be mindful of this. A good strategy is to use the heap like a LIFO buffer (i.e. always free memory in the reverse order that it was allocated).

Can anyone give me a sample code for allocating memory using malloc?

/* 
     * XC16 v1.31
     * PIC24FJ128GB606
     * MPLAB X IDE v3.65
     * Simulator Debugger
    */

#include <xc.h>
#include <stdio.h>
#include <stdlib.h>

volatile int i=0;
int main ()
{
    volatile int *ptr= malloc(100*sizeof(int));
    if (ptr == NULL)
        printf ("Cannot allocate memory\n");
    else 
    { 
        printf ("Memory Allocated successfully \n");
        for (i=0; i<(100*sizeof(int)); i++)
        {
            *ptr=i;
            printf ("Pointer value=%d \t i=%d\n",*ptr++,i);
        }
    }
    free ((void *)ptr); 
    return 0;
}

Also refer section "Standard C Libraries" under XC16 compiler directory, docs, "16-Bit_Language_Tools_Libraries_Manual.pdf".