Digital BW, The Print

I bought a box of Moab Entrada 5x7 cards (7x10 scored sheet).  I
cannot for the life of me get them to feed into my R2400.  I've
defined a custom paper size (tried both 7x10 "portrait" orientation
and 10x7 "landscape") and attempted to feed it in the thick paper
single sheet feeder in the back, the normal sheet feeder, and the
front feeder... all to no avail.  I get a message that says "media out
or not loaded correctly".

Which brings me to a general question about custom paper sizes: it
seems that once I define a custom size, there is no place in the Page
Setup or Print dialog to tell the printer where you will be feeding
that custom size page.  All of the normal paper sizes have options for
"sheet feeder", "manual roll", "manual front" etc. but with custom
sizes those aren't available.

If anyone has had success with the 5x7 cards I'd appreciate it if you
could share your method.

Thanks.