Have you tried the Application.PackagePassword
setter property?
Link to documentatation: http://technet.microsoft.com/en-us/library/microsoft.sqlserver.dts.runtime.application.packagepassword.aspx
This method is also mention in this SO question: How can I execute a package with a package password in SSIS through code?
NB according to that link you have to specify the password on the application BEFORE loading the package.