SSIS packages in 2005 are more confusing than 2000 DTS packages.
I'm working off memory here, but basically:
With SSIS (not SSMS) You first export the package from "MSDB" to file system such as C:\MSSQL\MySSIS_Packages\TableTransfer.DTSX
You open the 2005 SSIS package file "TableTransfer.DTSX" with Business Intelligence Development Studio.
Make your modifications, then save C:\MSSQL\MySSIS_Packages\TableTransfer.DTSX
Then, in SSIS, Import TableTransfer.DTSX back into SQL.
You can now execute or schedule the SSIS package with the modofications.
I'm sure someone else has a better explanation or example.