I have an Arab too, and she was seriously underweight when I bought her. Now she's fine but it took me about 4-5 months to see the results.
I would recommend free choice good quality hay, feeding her 3 times a day with whatever food she likes (I don't know anything about the US brands of food) as she will absorb the nutrients better when fed often and smaller quantities, add corn oil (it takes minimum 2 weeks before the horse begins to absorb the energy from oil) and remember max. 3 dl (don't know the American measures but it's about a small glass of water), if you give her more, you should add extra e-vitamin to her diet, plus a good vitamin-mineral product.
And make sure her teeth are okay.
Don't expect to see results immediately, it could take 3-4 months before it will show.