C# Microsoft.Office.Interop

Question

In a C# 2008 application, I am getting the following error message:"
Could not resolve this reference. Could not locate the assembly "Microsoft.Office.Interop.Excel, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c". Check to make sure the assembly exists on disk. If this reference is required by your code, you may get compilation errors."
I would like to ask the following questions:
1. The workstation I am installing the excel.interop has  excel 2010 installed on it. However the reference points to excel 2007.
Thus can you tell me which interop I want to use excel 2007 or excel 2010?
2. When I downloaded the interop for 2010, i tried to add the assembly reference to the visual studio 2008 solution file. However from the references in solution explorer, I selected add references and then I selected the .net tab. I could not find the interop reference.
Thus can you tell me if I am looking in the wrong place to add the reference? if not, do i need to need to work with interop for 2007? What do you suggest I do?