You need to either import the proper namespaces, like this:

#include <Windows.h>
#include <vcclr.h>
#include <iostream>
#include <vector>

#using <mscorlib.dll>
#using <System.dll>
#using <System.Management.Automation.dll>

using namespace std;
using namespace System;
using namespace System::Collections::ObjectModel; // you need this one
using namespace System::Management::Automation;

void RunPowerShell()
{
  // or simply use C++ auto!
  Collection<PSObject^>^ powObject = PowerShell::Create()->AddScript(gcnew String("get-wmiobject -query \"select * from win32_bios\""))->Invoke();

  for (int i = 0; i < powObject->Count; i++)
  {
    Console::WriteLine(powObject[i]);
  }
}

int main()
{
  RunPowerShell();
  return 0;

or just use the cool C++ auto keyword, like this:

auto powObject = PowerShell::Create()->AddScript(gcnew String("get-wmiobject -query \"select * from win32_bios\""))->Invoke();

And as a bonus, Visual Studio's magic will show the namespace for you if you hover the mouse around the auto keyword: