When you open an email that includes an attachment, you should see an icon at the bottom of the message that represents the attachment (for a WM5.0 device, you'll see the icon in the Message header).  Press & hold your stylus on the attachment icon until a popup menu appears.  Select "Save As" and save the attachment to your Pocket PC.

Then you can use your Pocket PC's File Explorer application to navigate to the file and open it.