codice:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>  
#define S 10
#define K 15

typedef struct indici {

  int primo;
  int secondo;

} indici;

void printMatrix(int matrix[S][K]){
 int i,j;
  for(i=0;i<S;i++){
    for(j=0;j<K;j++){
      printf("%d ",matrix[i][j]);
    }
    printf("\n");
    }
}

indici insertMatrix(int matrix[S][K],int rand1,int rand2){
  indici z;
  matrix[rand1][rand2] = 1;
  z.primo = rand1;
  z.secondo = rand2;
  return z;
}

int main ()
{
  srand((unsigned)time(NULL));
  indici z;
  int mat[S][K];
  int i,j;
  for(i=0;i<S;i++){
    for(j=0;j<K;j++){
      mat[i][j]=0;
      printf("%d ",mat[i][j]);
    }
    printf("\n");
  }

  int n1,n2,count=0;
  do{
  n1 = rand() % S;
  n2 = rand() % K;
  printf("\nNumeri Random: %d %d\n",n1,n2);

  z=insertMatrix(mat,n1,n2);
  printf("Matrice:\n");
  printMatrix(mat);
  printf("Indici Funzione: %d %d\n",z.primo,z.secondo);
  count++;
  }while(count<15);
  return 0;
}
Scusa ma non basta questo codice??

Al massimo aggiungendo un controllo per vedere se una cella è già occupata puoi fare
codice:
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <time.h>  
#define S 10
#define K 15

typedef struct indici {

  int primo;
  int secondo;

} indici;

bool checkFree(int matrix[S][K], int x, int y) {
    return !matrix[x][y];
}

void printMatrix(int matrix[S][K]){
 int i,j;
  for(i=0;i<S;i++){
    for(j=0;j<K;j++){
      printf("%d ",matrix[i][j]);
    }
    printf("\n");
    }
}

indici insertMatrix(int matrix[S][K],int rand1,int rand2){
  indici z;
  matrix[rand1][rand2] = 1;
  z.primo = rand1;
  z.secondo = rand2;
  return z;
}

int main ()
{
  srand((unsigned)time(NULL));
  indici z;
  int mat[S][K];
  int i,j;
  for(i=0;i<S;i++){
    for(j=0;j<K;j++){
      mat[i][j]=0;
      printf("%d ",mat[i][j]);
    }
    printf("\n");
  }

  int n1,n2,count=0;
  do{
  n1 = rand() % S;
  n2 = rand() % K;
  if(!checkFree(mat, n1, n2))
    continue;
  printf("\nNumeri Random: %d %d\n",n1,n2);

  z=insertMatrix(mat,n1,n2);
  printf("Matrice:\n");
  printMatrix(mat);
  printf("Indici Funzione: %d %d\n",z.primo,z.secondo);
  count++;
  }while(count<15);
  return 0;
}