I cannot complete the mission where i have to scan people and arrest them if they're suspicious. I scanned everybody, but i don't know how to use the Scanner correctly, as everybody has the blue text. And from the arrest part, how exactly i do that?

1. If you find someone, he'll become handcuffed, this means he's arrested.
2. You just have to keep trying. Sooner or later you'll find someone. It's pure luck.

and how i can see if he is a rebel? The name will be red? Or what?

Exactly