See: MessageQueue.Formatter.

To de-serialise messages off the queue for a program that I wrote for a
client, we did something like this:

// Declare and setup the message queue
// Set formatter
messageQueue.Formatter = new System.Messaging.XmlMessageFormatter(new Type[]
{typeof(string)});

// Get a message off the queue (myMessage is System.Messaging.Message)
myMessage = messageQueue.Receive(new TimeSpan(0, 0, m_timeout),
messageQueueTran);

// Get the body of the message
string msg = (string)myMessage.Body;

In the end, our "msg" contained XML that was then parsed and passed into
another system, but it sounds like this is what you are attempting to do.

Hopefully this helps.