There is possibility that the old card doesn't support GL_FUNC_ADD_EXT. I have currently no checks for card capabilities, but it is possible to use this:
Code:
if nx.GLInfo('GL_FUNC_ADD_EXT') then // Card supports it...
or nx.GetEXT to get list of all extensions in 1 string, for parsing many extension at the time.

But i made a little code reordering because it was quick. That extension is only used for SubBlend, but blending was general purpose function for both. Now this extension will not be used with just addblend at all, because it is not needed. (It is needed if addblend is going to be used after subblend.) Try new SVN version. I did quick test on existing blending demos, and everything seemed to be in order still.
(This doesn't make sub-blending work on those old, or should i say ancient, cards though. They are doomed to live without it Unless custom fragment shader can be used...)