The Stanford Encyclopedia of Philosophy contains the following interpretation of the Argument of Illusion in the context of the problem of perception:
In an illusion, it seems to S that something has a sensible quality, F, which the ordinary object supposedly being perceived does not have. When it seems to S that something has a sensible quality, F, then there is something directly presented to S which does have this quality. Since the ordinary object in question is, by hypothesis, not-F, then it follows that in an illusion, S is not directly presented with the ordinary object supposedly being perceived. The same account of experience must apply to both veridical and illusory experiences. Therefore, in veridical experience, S is not directly presented with the ordinary object supposedly being perceived. If S is not directly presented with the ordinary object supposedly being perceived in veridical experience, S is never directly presented with an ordinary object.
I do not quite understand the link between us not being directly presented with the ordinary object in illusions and its case in all of our veridical experiences. The source further mentions the Common Kind Claim, in which "for any veridical perception of an ordinary object, we can imagine a corresponding illusion or hallucination which cannot be told apart or distinguished, by introspection, from the veridical perception".
How can the common kind claim then be used to establish the conclusion that we are never directly presented with ordinary objects, since the common kind claim merely states that we can hypothetically come up with an illusionary experience in which we are presented with the same set of sense datum as in the veridical experience? For instance, when I perceive a red flower, what kind of corresponding illusion can I imagine which cannot be told apart from veridical perception? I understand that when I look at a straw submerged in water, my sense datum is of a bent straw which is an illusionary experience. However, when I perceive a red flower, am I not directly presented with an ordinary object of a flower that is red?