I've been stuck for 4 hours in this problems. I just watched the tutorials of "modeling modular game assets" and tried this.
My problems is I just select an object and go to the texture paint mode and add the diffuse texture and paint some color.
After that I selected the another objects and go to the texture paint mode. At that time, I get missing "Add paint slot"
I'll very thank if you say how to solve this problems.