Hi Sylvia, you may have left it too late for before Xmas.
Visa procedures have slowed recently, but that is more for the Sahara, not the north.
However last year Americans in my group got visas in 8 days door to door from Algerian Visa Center in Washington (see here
This was with an invitation (or CdH) which I assume you will also need.
I'm sure it may have happened, but I can't recall anyone getting an Alg visa in Tunis - certainly not in a few days.
Normally it must be done in your country of residence.
No visas at the border (for Brits at least) since the 1980s.
I would sooner look for reliable info on active travel forums than places like ehow which looks like it was not written from first hand experience and is a couple of years old at least.
Have you looked on Thorn Tree? Many try to visit north Alg on there.