Can you try on a newly created career profile?
I re-downloaded the trainer and have tried the career mode difficulty option on both an existing career and new career. I don't get the error message box popup any more, which I did before (not sure if there has been an update to the trainer?)
The new career - it detects the current career level (I set this to legend), and when I press num pad 2, I get the activated sound. When I go in to nets mode though, the level is still set at legend - so it seems to have no effect on the difficulty. The only thing it does do is make the batsman model 1/2 size which people have reported before.
On my existing career - it doesn't seem to detect the current difficulty. It has automatically progressed to Pro due to gaining skill points, but the trainer detects it as Rookie (I think I started at this level to get the hang of the game). Again, I can activate the option and hear the sound - but it doesn't change the difficulty in the nets.