an error "expected unqualified-id before string constant" when compiling

Check your indentation.
This flavour of C++ doesn't allow function declarations inside other functions.

You aren't supposed to check your indentation, more re-do it so you can read your own code easier.

Maybe you are unsure because they are macros, but ISR(){ //... } is a function definition and must go at file scope like loop(), rather than in loop() itself ( nested ).

//Should not be declared inside a function
ISR(TIMER2_OVF_vect) {

   PORTD=(sinetable[j++]);
   TCNT2=455;
   if(j==32){
     j=0;
   }
 }