The fastboot install method doesn't require root, and your bootloader is already unlocked. So all you need to do is install fastboot on your mac and use that to flash the recovery image.
So, step-by-step (but I'm doing this from memory, not testing):
* Download the fastboot program to your Mac from here:
https://www.htcdev.com/process/legal_fastboot_mac. That will download a zip file (I did test that bit!). Move it from your downloads folder to wherever you'd like to put it, then unzip it. This will then leave you an executable called "fastboot-mac" (I rename it to "fastboot", so that for me the commands look the same as on Linux or Windows, but I'll use the name "fastboot-mac" here).
* In the terminal, cd to wherever you've put the fastboot-mac binary. I'll also assume that you've copied the recovery image to the same directory for simplicity. I'll refer to the recovery image as "recovery.img" - replace that with whatever the actual filename is when executing commands.
* Put the phone in fastboot mode and connect via usb.
* Test the connection by typing in the terminal
./fastboot-mac devices
This should respond with the serial number of your phone. If you get that, you're good to go. If not, type
./fastboot-mac
and it should respond with a list of options. If that happens but the previous command doesn't work let us know what error messages you got (also check the phone is in fastboot mode, and try with a different USB port/cable just to be sure).
* Assuming all of that is working, you can then flash your recovery (TWRP or stock) using the command
./fastboot-mac flash recovery recovery.img
It should be quite quick, but don't interrupt it if it takes longer than you expect.
And that should be all you need. If you use TWRP then it's probably simplest to boot into recovery mode and use its menu to reset (it should be fairly obvious: look for either the words "wipe" or "reset"). If you use the stock recovery you can try through the Android setting menu (which should work with stock, I've literally never used it so don't know whether it plays well with a custom recovery - I always would use recovery directly, but the stock recovery is more of a faff). If you use the stock recovery and want to do it from recovery mode you will probably get a black screen with a red triangle (possibly a droid on its back, but I think HTC normally use the red triangle). To get the actual recovery menu you need to press power and volume up together (from memory - I have mostly used custom recoveries), after which you will find options to just wipe the cache or do a factory reset.
Good luck!