Here is some sample code that demonstrate how to access the selection in the property grid. Note there can be multiple objects selected, not only one:

IVsMonitorSelection selection = (IVsMonitorSelection)yourSite.GetService(typeof(SVsShellMonitorSelection)); // or yourPackage.GetGlobalService
IVsMultiItemSelect ms;
IntPtr h;
IntPtr pp;
uint itemid;

selection.GetCurrentSelection(out h, out itemid, out ms, out pp);
if (pp != IntPtr.Zero)
{
    try
    {
        ISelectionContainer container = Marshal.GetObjectForIUnknown(pp) as ISelectionContainer;
        if (container != null)
        {
            uint count;
            container.CountObjects((uint)Microsoft.VisualStudio.Shell.Interop.Constants.GETOBJS_SELECTED, out count);
            if (count == 1)
            {
                object[] objs = new object[1];
                container.GetObjects((uint)Microsoft.VisualStudio.Shell.Interop.Constants.GETOBJS_SELECTED, 1, objs);
                object selection = objs[0]; // selection is here
            }
        }
    }
    finally
    {
        Marshal.Release(pp);
    }
}