Cant see Pico in Mac OSX

tried a Freescale FRDM KL25Z board and that is working as mass storage OK

Yes, but I think they also have a comms port (at least the ST mbed devices too) that probably wouldn't have been appearing.

I've been doing a bit of poking around - I even tried making a .kext file for Espruino, but I get the same Failed to find the CDC driver error.

It's probably a bit much, but have you considered just upgrading to Yosemite? I think it's a free update now.