how do i turn on pure cab in 3.3?

I am not in front of my Kemper ATM, however, from memory, it's in the Output Section.

Confirmed - it is in the Output Menu.

It has been there since OS 3.0 or so....

Since OS 3.1.4.11421 and before that in Artist Mode.

Quote from jzucker

and why isn't it in the manual?!?

You can find the relevant info in the reference manual.

Quote from hjscheffler

You can find the relevant info in the reference manual.

If there's a 3.3 addendum, I'm guessing that jzucker had looked at this instead of the full-blown one.