Preliminaries: Make sure you have the Data Analysis ToolPak loaded.  

• Check to see if you have the Data Analysis ToolPak
  • On the main menu, select Tools.
  •  Near the bottom, look for Data Analysis.
• If it is there, skip to Entering Data.
• If it is not there, then you must "install it."  Install the Data Analysis ToolPak
  • On the main menu, select Tools> Add-Ins.
  • Mark Analysis ToolPak. Click OK.
  • Verify the procedure worked by selecting Tools. Data Analysis should be near the bottom of the list.

  *If Analysis ToolPak is not listed in the Add-Ins dialog box, click Browse and locate the drive, folder name, and file name for the Analysis ToolPak add-in, Analys32.xll - usually located in the Microsoft Office\Office\Library\Analysis folder - or run the Setup program if it isn't installed. (This last step is courtesy Microsoft help menu.)