First, try refreshing the repository list in application manager. This should normally correct the problem. Alternatively, you may do an 'apt-get update' at the command line to the same effect (but try through the application manager first, I'm not certain exactly what the AP does differently from apt). After that, do an 'apt-get clean' at the command line, to be sure the files are re-downloaded at the next step. Then, try installing again.