You can only add an attachment using the Custom HTML option.
Click the CREATE NEW CAMPAIGN button on the Dashboard:
Give your message a name for future reference (recipients will NOT see this):
Select your recipients - if a subscriber is on more than one list, they will only receive one copy:
Choose BUILD FROM SCRATCH:
Then choose the CUSTOM HTML option:
Complete your sender details:
If you are copying/pasting custom HTML code from another program, you can use the handy "split-screen" to work in code on the LEFT and see the rendering on the RIGHT.
You can also toggle to the VISUAL EDITOR:
In both platforms, you'll see a PAPERCLIP icon near the top RIGHT to add your attachment:
For best practices, we highly recommend linking to documents (rather than attaching them) in your mailing that are hosted elsewhere (DropBox, your website, Google Drive, etc.) - this will ensure delivery of your message to your recipients.