Let's see, can you provide screenshots of what you're doing?
For now, make sure you've selected mesh mode:
Then, Add mode's strenght (green arrow) determines the percentage of influence you will have when you stroke (red arrow) on the vertexes with the brush.
While Direct mode allows you to 1. select a vertex by clicking on it 2. select a bone whose influence you want to modify 3. move the Weight slider and therefore change the influence.
I hope it helps figure out what went wrong!
