Do you want to put weight on because you think your labbie is underweight (as in actual kilos) or is she skinny? I tend to ignore the actual weight and go more by the touch test as actual weight varies so much on other factors. If she's active, got decent muscle tone and you can't actually see her ribs I would not worry. I've never had this problem with mine but I do try to keep them on the light side for the sake of their joints (and we do agility). If you're really worried I'd speak to your vet, if I were you, in case there is an underlying problem.
Thanks for your reply, I want to put weight on riva because I would like to see her with a bit more weight on her, but not too much. She is roughly 2 years old, and am feeding her on Gilpa Supa valu mix.