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("<MyProjectNamespace>.Properties.Resources", typeof(<any type in the assembly>).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));
Try using ResourceManager in the System.Resources Namespace. Obviously, it depends a bit on what you are trying to retrieve, but mine looks like this:
ResourceManager.GetString("String1", resourceCulture);