I cannot open the attachment in my email
E-tickets are sent by email in PDF format. So if the PDF won’t open, then make sure you have a PDF viewer installed and then you shouldn't have any problems with it.
There is a chance that you might have received a Passbook file (.pkpass) if you checked the corresponding option in your user preferences. A Passbook file can only be opened using the Passbook application on a iPhone. Though do not worry! This is only optional, you can choose to deactivate this setting and just open the PDF instead. Plus, you may have just received a reminder notification and not a ticket. We also send Passbook files for print-at-station tickets. So the file does not count as a ticket at all and instead you still need to print your tickets yourself.