1. What's the source of this Conrad paraphase?
Christensen is almost certainly referring to the quote you've found. The source of that quote is the Preface of Conrad's The Nigger Of The "Narcissus", which you can access online (as html) at Project Gutenburg.
2. What does he mean?
Conrad uses his Preface – "which is simply an avowal of endeavour" – to explore the nature of creative writing. He opens it boldly:
A work that aspires, however humbly, to the condition of art should
carry its justification in every line. And art itself may be defined
as a single-minded attempt to render the highest kind of justice to
the visible universe, by bringing to light the truth, manifold and
one, underlying its every aspect. It is an attempt to find in its
forms, in its colours, in its light, in its shadows, in the aspects of
matter and in the facts of life what of each is fundamental, what is
enduring and essential—their one illuminating and convincing
quality—the very truth of their existence.
The artist, he says, speaks to "the subtle but invincible conviction of solidarity that ... binds together all humanity". More explicitly, he says "Fiction—if it at all aspires to be art—appeals to temperament" and goes on to say that if this appeal is to be effective, it must be conveyed through the senses.
In the fifth paragraph he cuts to the chase (I've put in bold the key words Christensen is referring to):
The sincere endeavour to accomplish that creative task ... is the only valid justification for
the worker in prose. And if his conscience is clear, his answer to
those who in the fulness of a wisdom which looks for immediate profit,
demand specifically to be edified, consoled, amused; who demand to be
promptly improved, or encouraged, or frightened, or shocked, or
charmed, must run thus:—My task which I am trying to achieve is, by
the power of the written word to make you hear, to make you feel—it
is, before all, to make you see. That—and no more, and it is
everything. If I succeed, you shall find there according to your
deserts: encouragement, consolation, fear, charm—all you demand—and,
perhaps, also that glimpse of truth for which you have forgotten to
ask.
To put it in modern parlance: his response to those who want immediate gratification is that his task is, especially, to make them see [i.e. see the commonality of human experience that he has previously described], and in so doing, he will not only give them the gratification they want, but maybe also a deeper experience.
