I want to print to some 250gsm card stock and I understand my HP4M+ can do this but I can't work out how to do it.

Wikipedia says this:

Straight Paper Feed

A feature of the 4+, and 4si particularly dear to graphic designers was the straight paper feed. Unusually thick paper could be run through the printers without jamming by setting the output tray to "Lower (face up)" and having it come out the back of the printer.
I understand the card stock is fed in via the MP tray (the multipurpose tray (aka the Manual Feed slot) but if I open the back, I get a "13 Paper Jam" error message and am unable to print. I can't see any menu options in the control panel to allow this nor in HP Web Jet Admin.

Like many others before me, I have been unable to find a User Guide on the 'net (service manual, yes, user guide, no). HP no longer have it for download (if they ever did).

I'd really appreciate it if someone knows how and would tell me – or can supply a copy of the pages from the user guide.