I have a connection string to read an excel file from my C# project that looks like this..
String ConnectionString = \"Provider=Microsoft.ACE.OLEDB.12.0;\"
Other option would be to use a specialized library instead of creating a connection. Take a look on EPPlus, its an open source library to work with excel files in C#. It has worked very good to me.
http://epplus.codeplex.com/
And in this link you can see examples on reading excel files with EPPlus:
http://blog.fryhard.com/archive/2010/10/28/reading-xlsx-files-using-c-and-epplus.aspx