On the tablet, check that it's in 'USB File Transfer Mode".
By default, your phone (or tablet) will be in USB charging only mode. This means no data is going back and forth between your phone and your computer's USB port, and only the 5 volt power is active. That's good for charging the battery, but means you can't see the phone in any type of file or folder browsing software, and the Android File Transfer app for Mac computers will pop out an error. Every time.
What you'll need to do is go into your notifications, and tap the USB connection entry. You'll get a dialog that allows you to change from charging only (the default) to transfer files (MTP) (lets you copy files back and forth between your tablet and computer).
If you're already in 'USB File Transfer Mode' and you're still not seeing your Android device, see this FAQ article.