The compiler cannot determine which Application object you want to use, so you have to specify the fully qualified namespace. Try it this way:
Excel.Worksheet activeSheet = ((Excel.Worksheet)(Microsoft.Office.Interop.Excel.Application.ActiveSheet));
Learn how to use google. Answers to your questions can be easily found, and is faster than asking a question on a forum, and waiting for a suitable answer. I'm not saying this because I'm tired of answering your questions. I'm saying this because debugging and using google are the two MOST important skills you can use to develop your code faster.