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Just use the B&N app to read them.
Barnes & Noble eReader - Read 1 Million eBooks, Free Books, Newspapers & Magazines for iPad on the iTunes App Store
It's free, and that's what it is made for.
But where did you buy yor copy of "The Lost Symbol"? If you bought it from Barnes & Noble and you still can not open it, then you have an issue with them. If you did not buy it from Barnes & Noble, it is understandable that you can not open it. The Barnes & Noble app is not a "universal" reading app, it is only for reading material purchased from Barnes & Noble.
Then there is definitely an issue with the BN Reader and you should contact their support department.
(I promise, I won't tell you how it ends!)