Hi Susan,
Feedback is a tricky subject, that can be useful...or useless. It's common practice (at least in the US) to not provide any feedback when asked, because it opens up a company for a discrimination lawsuit. Most of the time you get nothing.
And when you do get something, it's hard to say if it's genuine or not. Tech companies cite vague words for feedback, like "he didn't have a good fit for our company culture." This may be a way to weasel around the legal problem above.
But if you think the feedback you got was credible, step outside yourself and analyze the situation. You didn't have a "technical fit" -- which sounds kind of like the BS above to me, but okay -- so why did they think that? Did they say something about it during the interview, like "You haven't done ___ in a while, how will you get around that issue?"
The salesman in me would, in the future, reverse it on the interviewer to catch them off guard. (This usually makes them respect you more.) If it's evident on your resume, I would use it for the answer to the inevitable "What's your biggest weakness?" I would say, "My biggest weakness? I haven't done ____ in a while, maybe in ___ years. However, I know that it won't be a problem because ___[insert reason why you can do the job here]___."
Finally, as Rich says, there are two ways to think about it, too. 1) What did I do wrong to make them not like me? and 2) Screw those guys, they weren't intelligent enough to give me a chance, I wouldn't want to work with the kind of person who think I'm not a good technical fit anyway.
I can't suggest doing what John D. D says: you don't need a course, tutoring, teaching, or to suck up to them with your GRE chemistry scores. You should thank them for the opportunity for an interview and for the feedback...and then move on to the next interview. You do have several lined up, right? :)
PS When I was interviewing I found Dave Jensen's article on feedback useful:
http://sciencecareers.sciencemag.org/ca ... t.a1400120
"The single factor that differentiates Nobel laureates from other scientists is training with another Nobel laureate." -- Sol Snyder