If you had a reference to the correct assembly already then you can probably set it back again. In your project right click on reference, add reference, then search for interop.excel on the top right of the window:
If the version 14.0.0.0 is found just enable it and disable the newer one. As you see I have the same situation with both of them on my computer.
If the search won't find the assembly you can download it here: http://www.microsoft.com/en-us/download/details.aspx?id=3508