Here is how to set variables in the package from code -
using Microsoft.SqlServer.Dts.Runtime;
private void Execute_Package()
{
string pkgLocation = @"c:\test.dtsx";
Package pkg;
Application app;
DTSExecResult pkgResults;
Variables vars;
app = new Application();
pkg = app.LoadPackage(pkgLocation, null);
vars = pkg.Variables;
vars["A_Variable"].Value = "Some value";
pkgResults = pkg.Execute(null, vars, null, null, null);
if (pkgResults == DTSExecResult.Success)
Console.WriteLine("Package ran successfully");
else
Console.WriteLine("Package failed");
}