Physics lesson time! If you want to skip the physics, a solution to your problem is under this section:
When objects are pressed against something (like when an object is “pressed” against the ground by gravity) that something (ie, the ground) exerts a force back called the “normal force.” This is why objects don’t fall through the ground. The normal force is basically perpendicular to the surface (see attached image).
For example, when a ball is just laying on the ground, the force of gravity and the normal force are equal, therefore the ball doesn’t move.
The reason your object “slides” on slopes is because the force of gravity is creating a normal force that isn’t straight back up, but is perpendicular to the ground. Since there’s no force to counter-act the horizontal part of that “diagonal” force, your object
starts to slide. Not as fast as gravity, but pretty fast.
Friction is usually what keeps objects on slopes from sliding down. The force of friction is related to the normal force.
The way to stop your object from sliding is to give your ground a large enough amount of friction to counter-act the normal force. You can do that by assigning a material to the ground object, clicking the “DYN” button, and then cranking up the friction.
If you’re using Force to move your character, this will make it more difficult for your character to move up the slope, and he’ll probably go up it slower. I’d recommend using the Servo motion controls, they’ll give you a better result anyways.
Let me know if you have any questions