how do you trigger the approach ?

I mean once I remember that I got the option to tell me when to flap and when to use the landing gear. 

I tried approach phase but it did not trigger the flaps etc...

You mean the "Approach Checklist"?

Or do you mean like "Descent Mode" in Voice Control so your voice commands will be able to trigger approach flaps and landing gear.

I suspect the later.

Make sure you're not skipping the cruise phase events asking the FO is he's ready for the approach brief.

That's what puts FS2Crew into Descent mode.

I think this just answered the issue I had on my last flight. I didnt do the approach briefing and so my FO decided to put on his noise cancelling headset and take a nap until the end of the flight. Now I know what I forgot and how to avoid it but maybe the triggers should be a little more realistic. I think on a real flight the FO would still be performing his duties even if the captain messed up at some point. Maybe when we start the descend mode in the FMS the FO should say something like "Hey captain, I think you forgot the approach briefing", but he should still be doing his stuff when we get to 10000ft. 

Good suggestion.  We'll consider that.,

I suggest keeping the "Voice Command Assist" panel visible.

That way you can't really miss anything.

Cheers,