If you really think she needs to gain a bit, I'd use Satin Balls:
10# hamburger meat
1 jar wheat germ
1 lg box of oatmeal (uncooked)
1 1/4 Cup vegetable oil
10 eggs
10 sm pkgs unflavored gelatin
1 1/4 Cup unflavored molasses
A pinch of salt
1 lg box Total cereal (2lb's)

That's a huge recipe. You could cut it down if you don't have room to freeze the extras. I freeze it in small ziploc baggies. Just take out of the freezer and hand it to the dog. I've never had a dog refuse to eat them. I used them alot when Paxil was battling cancer.

I don't think she sounds dangerously skinny though. I have several dogs that I can see vertabrae or ribs very easily, mainly the long houndy types(Anvik, Ozzy, Mac..). There is lots of evidence that slightly skinny dogs live longer, healthier lives than their overweight counterparts(just like humans!). As long as you aren't see any other concerning symptoms, like lack of energy, poor coat, ect, I wouldn't worry too much about a couple pounds.