Also after activating my 'boris' there's still a 'X' on all the transitions.
Unless you have purchased the full BorisFX Continuum Suite (which includes transitions) or the Transitions Unit from BorisFX, the only licensed Boris FX that comes with Vegas Pro and Vegas Post is Primatte Studio, a single FX within the huge BCC Suite - any FX other than Primatte Studio will appear with a red 'X' because those other FX are not licensed.
Have a look at the following BorisFX video from a few years ago about how to install a licensed BCC FX without installing everything (view from around 02:45):