CSV is not an Excel format; what a .txt is to a .doc (Word document), a .csv is to a .xls (Excel document).
Excel can open CSV documents though. Open Excel, then from within the program open the CSV.
Alternatively, do as Gamanit suggested. If the document was emailed to you, don’t do this from within your email program (Outlook, etc.); do it from the file location.
Once you have it open in Excel, you can always Save As… an Excel document.