Most of the threads I\'ve read about this question answer that you just have to access them like this:
.Properties.Resources.
You should create a ResourceManager instance to open a resx file like this:
ResourceManager resources = new ResourceManager(".Properties.Resources", typeof().Assembly);
string myString = resources.GetString("myString");
Bitmap myBitmap = resources.GetObject("myBitmap") as Bitmap;
If they are the resources of a form you can also get them as following:
ResourceManager resources = new ResourceManager(typeof(Form1));