The bootloader on the HTC One XL was unlocked unofficially with a hack by XDA Recognized Developer DesignGears, but as is usually the case with hacks for Android devices that require a computer to work, they are usually only supported on Microsoft Windows, at least initially. Users of Mac computers have to then wait till a particular hack gets ported to OS X, or try to gain access to a computer that runs Windows.
But the wait is over for Mac users who own an AT&T or Rogers HTC One XL and would like to unlock the bootloader on their phone, as XDA Senior Member jnichols959 has ported the bootloader unlock method to Mac and released the Mac Bootloader SuperCID Unlock Script (works only on Mac, not for Windows or Linux users). The script unlocks the bootloader as well as gives you SuperCID access so you can flash custom ROMs and any stock firmware regardless of carrier.
Head over to the official instructions page on the XDA forums and follow the mentioned steps to unlock the bootloader on your AT&T/Rogers HTC One XL using your Mac computer. Let us know how it works for you, in the comments below.