How to boot into the bootloader when you have ClockworkMod? I want to upgrade the radio and I'm not sure how to boot into bootloader.

When I try to power the phone using VolumeDown+Power, I do get into the ClockworkMod Recovery, but it seems that this is not what I was looking for because on PC the command fastboot flash radio radio.img is still &lt; waiting for device &gt;.

In case it matters, this is a permarooted + hroot + ClockworkMod, HTC Desire HD.
Make sure your phone is powering off completely before you try to boot into the bootloader. For a DHD you might need to go into Settings &gt; Applications and uncheck the "FastBoot" option in order for it to power down completely (source).
If #1 doesn't help, try turning your phone off, then pulling the battery. After you've pulled the battery and waited a few seconds, hold the volume down button and put the battery back in the phone. Then continue to hold volume down while you power up the phone and it should land on the bootloader.
If #1 and #2 don't work (or you're lazy, since I think this is the easiest way), plug your phone into a computer that has adb on it. Make sure USB debugging is enabled, then simply enter adb reboot bootloader from an adb terminal. This will (unsurprisingly) reboot your phone into the bootloader.