It doesn't work because the object that findP references is not in the
combobox.  It was created the same as one that is there, but it is a
different object.  You must use the same object.

Since you know you want to select the 5th one, I would use:
   Combobox1.SelectedIndex = 5-1;

If you don't know ahead of time which one you want:

   int selIndx = -1;

   for (int i = 1; i < 10; i++)
   {
       Person p = new Person("JDoe" + i.ToString());
       Combobox1.Items.Add(p);
       if ( ... )
           selIndx = i - 1;    /* Select this one */
   }
   Combobox1.SelectedIndex = selIndx;

Both examples assume that the combobox is empty before the loop.
Another possibility is to save p in another variable inside the loop
and set SelectedItem to the saved variable.  I would presume that
using SelectedIndex would be more efficient than SelectedItem, since I
figure SelectedItem must sequentially examine each item in the list.