namespace name 'Excel' does not exist in 'Microsoft.Office.Interop.Excel

Your title and the body of the question contradict to each other.

From your question title, it's clear that you expected Microsoft.Office.Interop.Excel.Excel to exist. (Read your own title carefully.) You can easily make sure that such type does not exist:
http://msdn.microsoft.com/en-us/library/microsoft.office.interop.excel%28v=office.14%29.aspx[^].

You just messed up things. And then you though that Excel should be in Microsoft.Office.Interop, as it is expressed in the body of the question. Of course it exists, but this is always a namespace, not a type. You might want to work with the types, like Microsoft.Office.Interop.Excel.Application, as well as any other types.

—SA