> I have set the listbox to be of type int in the property panel.

I don't know what you mean by this since the standard listbox has no such property in Visual C++, though in VB you can set a listbox property data format as Number with 0 decimal places.

In C++, convert the integer to a string, then call InsertString():

int nIndex = 2;
char* pInteger = "100";
char pIToA[10]; // make it large enough to hold the largest integer plus a null byte
itoa( pInteger, pIToA, 10);
listbox.InsertString( nIndex, pIToA );