To access environment information from within an activity, you'll need to use the System.Activities.Hosting.SymbolResolver class.

Here's the code to access the symbol resolver from within an activity:

public sealed class SymbolUser : CodeActivity
{
  protected override void Execute(CodeActivityContext context)
  {
    SymbolResolver symbolResolver = context.GetExtension<SymbolResolver>();
    MyObject obj = symbolResolver["CustomObject"] as MyObject;
    Console.WriteLine(obj.MyString);
  }
}

public class MyObject
{
  public int MyValue { get; set; }
  public string MyString { get; set; }
}

Here's the code to add the symbol resolver to the workflow application:

 

class Program
{
  static void Main(string[] args)
  {
    WorkflowApplication WFApp = new WorkflowApplication(new Workflow1());
    SymbolResolver symbolResolver = new SymbolResolver();
    WFApp.Extensions.Add(symbolResolver);
    symbolResolver.Add("CustomObject", new SymbolUserActivity.MyObject() { MyValue = 1, MyString = "TestString" });
    WFApp.Run();
    Console.ReadLine();
  }
}