8045 - how to engage 4wd

We recently purchased this tractor, and have finally set it all up, went to put it to work, needs to work on a beach, but doesnt seem to engage 4wd. we have a workshop manual, but that doesnt explain much, we tried every lever in the cabin, but still nothing, back tyres are spinning on sand, front not pulling like they should, can someone explain how to engage the 4wd?

There is a round knob down below the left hand side of the drivers seat on the cab floor pan upright section, just pull this fully out to engage 4wd