Pop up ads are not used to spread Mac ransomware. However, malicious emails have been playing an important role when spreading such viruses, so make sure that you stay away from them.
How could you spot the malicious email message among the legitimate ones? First of all, you should check the sender. If you don't know him/her, you should ignore the email message or, if you are still curious about it, contact the sender and ask about the email. Secondly, make sure you read the email body carefully and look for grammar/spelling mistakes. The most of malicious email messages are filled with them, so you should definitely ignore the ones that are written with mistakes. Finally, check the attachment before downloading it to your computer.