Just for clarity, msdb.dbo.sysdtspackages90 is where Pakages were stored in SQL Server 2005. In SQL Server 2008 they are stored in msdb.dbo.sysssispackages.

Here is a PowerShell script already written that needs only a minor tweak or two to wrk for you in your environment to get files from the Packages in the server:

Export all SSIS packages from msdb using Powershell