Per Books Online "The COMPUTE and COMPUTE BY clauses are provided for backward compatibility." so use those in new development at your own risk. They do not exist in SQL Server 2012.

If you still want to use COMPUTE BY, know that it returns a second resultset. You can capture both if you execute your query to Fill a DataSet within a Script Task. You can then iterate over the DataSet to write your Excel file, but all of this would be custom .NET coding and you wouldn't really be using much of the built in SSIS functionality at that point.