does the red LED serve as the error message?
The red LED indicates there's power.
If the firmware and bootloader are loaded successfully, the green ACT light will flash twice. This happens immediately when you plug it in. Booting the OS kernel then proceeds and ACT may continue to flicker as the OS does stuff (subsequent to the two flashes, ACT indicates activity involving the SD card). At this point the kernel should be spitting messages to console so any errors will be reported there.
If there is an error before the kernel is loaded, the ACT blinks three or more times, meaning, according to this:
- 3 flashes: start.elf not found
- 4 flashes: start.elf not launched
- 7 flashes: kernel.img not found
- 8 flashes: SDRAM not recognised. You need newer bootcode.bin/start.elf firmware.
No SD card is not exactly an error, though; in that case the ACT light will never flash or glow. If it does not and there is an SD card in, the card reader may be broken -- or maybe you did not quite push it in all the way ;)