You don't. As mentioned in your other thread on this topic you use the selected index.
As for adding multiple items, you could use a loop:
int i = listboxRadStr->SelectedIndex;
// No item selected
// First item selected (1)
// Second item selected (2)
C++ with managed extensions has not been widely adopted and samples are hard to come by, even in the MS documentation. You may want to look into switching to C# for your CLR work.
for (int i = 1;i <= 12;i++)
As I have not used MC++, I can't guarantee that the code I have posted will compile.