Office 2007 Users
Click on Office Ribbon and press 'Excel Options' button
Press 'Add-Ins' and select 'COM Add-ins' from Manage options and press'Go'
If 'SpreadsheetWEB Addin' is not enabled, select the checkbox and press OK
Office 2010 Users
Press File from the main menu and click 'Options' button
Press 'Add-Ins' and select 'COM Add-ins' from Manage options and press'Go'
If 'SpreadsheetWEB Addin' is not enabled, select the checkbox and press OK
Also see http://msdn.microsoft.com/en-US/library/ms268871(v=vs.80).aspx