One way to understand the basic argument you outline, which is advanced most famously by Thomas Nagel in "What is it like to be a bat?" and Frank Jackson in various papers about Mary the color-blind super-scientist, is like this:
1. If physicalism is true, then someone who knew all the relevant physical facts about a conscious being's experience (e.g., a bat or a person seeing red) should know what it is like to have those experiences without having had them (i.e., without experiencing sonar perception or without having seen red).
2. Someone who knew all the relevant physical facts would not know what it is like to have those experiences.
3. So, physicalism is false.
I think there are good reasons to reject both premises. Premise 2 looks like an appeal to ignorance. It does seem implausible that any amount of objective (or 3rd personal) information could allow someone to understand conscious experiences she has not experienced. But we do not really know what a physicalist theory of consciousness will look like, or what it would be like to have all the relevant information, including the theory itself. So, premise 2 may not turn out to be true.
Premise 1 is even more dubious. A physicalist theory should predict that only by being in particular physical (e.g., neural) states will one be in certain conscious states. The theory should explain why that fact of first-person access is the case. It should also be able to explain why a person (or bat) who is in a particular physical state is in a particular conscious state. But I don't see why we should expect physicalism to entail that the experiences themselves become "illuminated" from the outside.
Some will try to strengthen premise 1 to say that physicalism entails that all the possible facts are physical facts, such that knowing the physical facts entails knowing all the facts. I'm not sure what that means--that is, the use of 'facts' and 'knowing' seems to load more into the premise than a physicalist needs to accept.
(The literature on these debates is huge. If interested, you might start here: http://plato.stanford.edu/entries/consciousness/)