You can use THFloatTensor_get2d(input,i,j) to accese ipnut[i][j]. A general method  is to compute the index of the value by hand. In the 2d tensor case, for input[i,j]:
long offset = input->storageOffset+i*input->stride[0]+j*input->stride[1];
float val = THFloatStorage_get(input->storage, offset);