Those are XML namespaces and they can be a pain to use. Have a look at the Adobe documentation on Using XML namespaces.
Basically you have to get the namespace:
var mmsNS:Namespace = message.namespace("mms");
// alternatively for nested namespaces:
var mmsNS:Namespace = new Namespace("mms", " ... url of namespace ... ");
And then use it when you want to get the nodes that it is applied to:
var data:XML = message.@mmsNS::Results;
A shortcut if all of your xml is in a particular namespace is to set the default namespace:
default xml namespace = mmsNs;
edit: The XML namespace you are trying to access must be declared within the XML fragment:
<root xmlns:mms="http://example.com/mms">
<mms:someNode someAttr="someVal" />
</root>
Have a look at the w3c docs for XML Namespaces to ensure your document is well-formed (maybe even pass it through a validator).