Fit a Sub and Amp in an Octy 2 which is fitted with an MFD3? Are there any guides out there?

Sorry I am complete Noob when it comes to car audio, and I cant imagine the MFD3 having a sub out, which is the only way I would know how to wire one up.

There are generally 2 ways to hook up an active sub - via an RCA pre out direct from the head unit or via the speaker terminals via high level inputs. The amp takes a signal from the speakers and filters out the non subwoofer frequencies. Most enclosed active subs come with the wiring loom to achieve this, there are also line level converters with RCA connectors that convert the signal from the ordinary door speaker.

An example of one here: FLI Line Level Converter on eBay, also, Terminals Cabling, In-Car Entertainment GPS, Cars, Parts Vehicles (end time 06-May-09 12:38:11 BST)

One end to a door speaker, the other end to the subwoofer line in RCA jack - job done