Save the placemark in My Places, then right-click on it and choose Save As, then save it to a folder where you'll find it later. Start a new message and look down the page in Additional Options. You'll see a button marked Manage Attachments. Click on that and then Browse to where you saved the placemark and select it, then click Upload. When it's finished uploading click on Close This Window and write the text for your message, then you can post it.
That should do it, I'm sure if I've missed som,ething then someone else will point it out.