Hi -
1. You'd have to define BN_LONG yourself - presumably with a typdef.
2. I didn't mean the article as a complete, prêt-à-porter solution.
I merely thought it would give you useful pointers for developing your own solution. More like an "algorithm", than code you could simply cut/paste.
3. If you want an "off the rack" solution, however, I'd encourage you to consider GMP (as others have also recommended).
You might also want to consider Tom's Math library (also, along with GMP, mentioned in the article I cited):
http://www.freshports.org/math/libtommath/
http://libtom.org