This might be really simple to do but I just have no idea how... Can I sync my Mac to an analogue drum machine/sequencer (with MIDI out) so my Mac, which is running Reason, Live4 and GarageBand2, as the slave?

Its not a massive problem, but if it can be done, my life would be a thousand times easier!

The short answer is YES.

You must have a midi-interface of some kind.

Konfigure your drum-machine to send MTC to your midi-in input.
Konfigure your drum machine to send on a specific midi-chammel

Connect the USB-Cable to your Mac.
( your midi-interface is usb-based - right?)

In Audio/Midi setup make shure your Midi-Interface shows up
(you run Mac OS-X Panther - like right?)
IF NOT - install midi-drivers from your midi-interface manufacturer
Konfigure Audio/Midi to reflect your physical midi-setup
http://www.skatboet.se/skatboet/pics/midi.jpg

Configure Reason to accept external clock sync from your drum-machine
on the same midichannel your drum-machine is transmitting on.

Cheers: Dixiechicken