Make sure you have selected the PPD for the IR5570 (Format...Document...Basic Attributes, or press F4). Then click on paper handling. Set selction type to input tray if it isn't all ready. Then select Stack Bypass in for the input tray from the paper source section.
Alternatively, if using "manual feed" will work the default PPD does have COM-10 as a paper size, which I believe is a standard envelope. You should be able to set the paper size to COM-10 and the selection type to match paper size. This should force the printer to prompt for the evelopes to be loaded in the stack bypass.
Or use window envelopes...