How does goomanager work?
Though I wouldn't usually recommend device specific websites but since i'm not a One XL user I cannot verify its authenticity.
Next time when you ask questions about custom roms please mention whether you are already rooted, locked or unlocked bootloader(htcdev.com only unlocks your bootloader without giving you s-off). Try this to just get an idea of one of the methods to install a rom.

http://onexroot.com/one-x-roms/how-to-install-custom-rom-on-rooted-att-htc-one-xxl/

goomanager is a service used by developers to upload their roms into one location and users can check for available newer versions of existing roms. Follow the regular clockworkmod / twrp procedures to flash your rom.

You can always copy roms that you downloaded with your computer onto your phone.