Attachments in Outlook will display in different places depending on the message format. Rich Text will display attachments in-line with the message body; HTML and Plain Text will display attachments in a separate field under the Subject line.
From the main screen (not inside the email itself) in either Outlook 2003 or 2007:
Tools > Options > Mail Format > Message Format
Set this to either HTML or Plain Text. HTML will allow them to use custom colors and fonts in their messages; Plain Text will not.