The force should be a vector pointing in the direction you wish to apply the force. Multiply it by some magnitude. Example:
-- apply force in direction of game object with id "to"
local from = go.get_position("from")
local to = go.get_position("to")
local direction = vmath.normalize(from - to)
local force = direction * 10000
msg.post("from#collisionobject", "apply_force", { force = force, position = from })