Your primary problem is that you're missing an include:
#include <sstream>
Also, you have a typo in the function, and your return statement is downright crazy. With these fixes, it works:
#include <iostream>
#include <string>
#include <sstream>
string simpleOutput(){
stringstream ss;
int a = 50; // for testing purposes
int b = 60;
string temp = "Random";
ss << a << b << temp;
return ss.str();
}