ADB is working, fastboot is not (HTC Desire Z)

The adb devices command is working for me, the fastboot devices command is not (Mac OS X Mavericks, SDK downloaded yesterday, phone is a HTC Desire Z):

$ sudo fastboot -i 0x0bb4 -p 0x0c91 devices
$ fastboot -i 0x0bb4 -p 0x0c91 devices
$ fastboot devices
$ adb devices
List of devices attached 
HT0C5RT00804    device
$ adb shell
#

So fastboot doesn't list devices, whatever I try (sudo, or specifying -i and -p options), while ADB works just fine. I found vendor and product IDs via Mac OS X system information. ADB working seems to indicate that communication between my laptop and the phone does work fine.

How can I get fastboot to work?

Background info:

I'm trying to install cyanogenmod on a HTC Desire Z. Part of the procedure I'm following is described here: http://ift.tt/1nf4MFf

I get stuck at the fastboot oem rebootRUU step:

 $ fastboot oem rebootRUU
 < waiting for device >

(hangs, I gave up after 20 minutes), and I think that is due to fastboot not working. Any other ideas on how to get past this appreciated?