Thought i would do a write up as i had trouble with this.
The BMC CDA Kit comes as a universal kit, which means it isn't a direct fittement. (Which confused me as i didn't realise it!)
1.
Remove Old airbox. I unclipped the top half, then wriggled the bottom half out. then gave it a quick clean of any debris.
2.
Hose connection. Due to it being universal, you need to join the Kit with a hose. I got an 83mm hose from here:
http://siliconhoses.com/content.asp?inc=product&catalog=0022C&cat=0022C connect this to the engine side of the Kit, like so:
3.
Connect it back up. Then connect the hose back up, tighten the jubilee clips and your good to go.
Here is mine all connected up:
You get a better sound if you don't use the cold airfeed i've been told.