While reading your message that contains the attachments, you need to scroll to where the attachment is located (mine are typically all the way at the bottom of the message). Once your cursor is on the attachment name, you can click the thumbwheel and select Open Attachment from the menu. If you don't have that option, you may be trying to open an attachment type that is not supported. My device (7100t on BES) has the ability to open Microsoft Word, Microsoft Excel, PDF and ZIP files (at least -- I haven't tried anything else yet) and I did not need to install any additional software to get this support.

Hope this helps.