I have the cert as a
.cer file on my Mac, and my phone is connected via USB. How can I install the cert on my device?
Rename the file with an extension of .crt and place it in your SD card /download folder.
Put the SD card back into your device (or remount it, if internal) and navigate to SETTINGS > SECURITY & PRIVACY > INSTALL CERTIFICATE FROM SD CARD.
Hopefully that ought to do the trick, else try this online Certificate Installer tool - it gives you a link that you can navigate to using the Android web browser and install the certificate.