A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
When you did Start > Programs > Excel. Did that actually start a new instance of Excel. Check the View > Window > Switch Windows.
Do both Book1.xls and Namespacea.xlsm appear in the list? If not then that is the problem, you have 2 instances of excel running.
To create an addin you simply need to save the excel file as a .xlam file.
http://office.microsoft.com/en-gb/excel-help/add-ins-HA010351123.aspx
Cheers,