If you want to get your milestone on bootloader mode you've to power off the device and start it by pressing Power + DpadUp. This will bring your milestone on a black screen.

When the bootloader is ready to be flashed it shows the version - see the mbm page

To access the Fastboot mode on the Bootloader, use the “adb reboot bootloader” command via USB with the Motorola update kit installed on the PC. Usually, this mode is not available on the Droid. Unfortunately, although fastboot does work with the Milestone, none of the known fastboot commands seem to work (except develeper phones).