You need to read the associated article found at: http://blogs.msdn.com/b/alikl/archive/2011/06/05/how-to-request-swt-token-from-acs-and-how-to-validate-it-at-the-rest-wcf-service-hosted-in-windows-azure.aspx follow the steps to Configure ACS to Issue a SWT Token. The information you enter when completing the section "To configure a service identity for the REST web service" is what goes here.
If you are using a Symmetric key for your password then you need the client to request a token from ACS in a different way than the example. The following code is an example of what that request looks like and was taken from http://msdn.microsoft.com/en-us/library/hh674475.aspx. See the section "SWT token requests".
WebClient client = new WebClient();
client.BaseAddress = string.Format("https://mysnservice.accesscontrol.windows.net");
NameValueCollection values = new NameValueCollection();
// add the wrap_scope
values.Add("wrap_scope", "http://mysnservice.com/services");
// add the format
values.Add("wrap_assertion_format", "SWT");
// add the SWT
values.Add("wrap_assertion", "Issuer=mysncustomer1&HMACSHA256=b%2f%2bJFwbngGdufECFjQb8qhb9YH0e32Cf9ABMDZFiPPA%3d");
// WebClient takes care of the remaining URL Encoding
byte[] responseBytes = client.UploadValues("WRAPv0.9", "POST", values);
// the raw response from ACS
string response = Encoding.UTF8.GetString(responseBytes);