Depending on the browser, you will need to navigate to the menu bar and select the option 'Print...'.
Alternatively, you can use the keyboard shortcut keys of Ctrl + P to bring up the Print dialogue window.
Here is an example of the Print dialogue box within Google Chrome:
Please note: If you're unable to print the email or when you attempt to print the email the page is displayed as blank, this will most likely be because the sender of that email has applied read-only policies to the package and therefore printing, copying/pasting has been disabled. When viewing the email on Web Access, watermarks will be displayed over the message and attachments which will confirm whether it is read-only or not.