To download and print your ticket(s) PDF, you'll need to visit http://tickets.aftonshows.com and login to your account.
Once you're in the My Account section of the site, just click on the link called "Ticket History." There you'll be able to download a PDF of your ticket(s).
Once you download the PDF, you use a PDF-reader application, like Adobe's free Reader program (http://get.adobe.com/reader/) to open and print the ticket(s) on the computer you're using. Or you can transfer the PDF file to another computer (using email, flash drive, etc.) and print the ticket(s) from there.
If you're still having trouble printing your ticket(s), your name will also be on the "will-call" list for this show - with the number of tickets you purchased. If you just tell the person at the front door your name and that you purchased your ticket(s) online, they can verify this and grant you and your guests (if applicable) admission.