byte[] byteArray = Encoding.ASCII.GetBytes(test);
MemoryStream stream = new MemoryStream(byteArray);
You could have found this with google.
EDIT ==================
string test = "My test string.";
byte[] byteArray = Encoding.ASCII.GetBytes(test);
MemoryStream stream = new MemoryStream(byteArray);
It works just fine for me. (using .Net 4.0, but it should be good for any version of .Net).