C# fails when deployed to SQL Server

I have an SSIS package that uses a C# script to move some files.  It is in a script because of logic tied to the file movement.

It works great when I run from SSDT on my machine, but when I deploy to SQL Server it fails.  It fails on the 5th line...

        Dim SourceDir As String = CType(Dts.Variables("SourceFolder").Value, String)
        Dim ProcessingFolder As String = CType(Dts.Variables("ProcessingFolder").Value, String)
        Dim FilesPresent As Boolean = False
        '
        Dim Dir As New DirectoryInfo(SourceDir)
        'Dim FArray As FileInfo() = Dir.GetFiles()   '''this breeaks my package when deployed

I believe this has to do with permission.  How can I run a package under another account?

Any help is appreciated, thanks

Mike