OK, I changed my clothes, entered the park and poisoned the tea, but my target (Red Dragon negotiator) is frozen in the ground, he is talking with the other guy, but both are static like statues. Is there other way of getting Silent Assassin rating in this mission besides poisoning the tea?

Have you simply tried going to one of the exit points after poisoning the tea?

There is another option to get Silent Assassin rating by sniping (actually from two locations). I don't want to spoil anything unless you want to know.

I tried this. A message telling me the objectives are not complete appears when I go there. The building with a yin-yang sign? How I get rid of the guard near the ladder? I've tried to sedate him, but other guards saw. =)

The explosive solved my problem. =)
The building that I mentioned earlier (image attached) wasn't very useful at the start, since I couldn't aim the Red Dragon negotiator's head up there.
I planted a explosive in his car, climbed the ladder, fired a bullet near the Blue Lotus negotiator, and activated the bomb when the target was fleeing in the vehicle. Good thing this game allows you to choose different ways of doing the missions! The feeling you get after completing a mission after many failed attempts is great! =P