I know that I shouldn't be one to talk because I am scared of gaining weight too. Because I am not as underweight as you, they are allowing me to maintain my weight for a while until I get used to eating more and making progress in counseling. I have to record everything I eat and when though and go in for bi weekly weigh ins. If I lose weight or if I show a trend of deviating from the plan without a good reason, my dietician will make a recommendation for hospitalization. Quite frankly, I am suprised that for you that they didn't already recommend that, but perhaps they are giving you a chance to gain weight on your own, hence your post.
I don't know how your metabolism is doing, but I am not gaining any weight eating 1600-1800 calories/day, which shocks me because it seems like so much food. I am always convinced that I gained lots of weight for my weigh in, but I am still in the "weight range" (2 pounds on either side of my weight my first day w/ the dietician) after 2 months.
My husband's friend recommended weight gainer shakes to me in order to gain weight. To me though, that is the ultimate "unsafe food". Being forced to maintain my weight though, has made me more comfortable eating "unsafe foods" like fast food burgers occaisionally and "less safe foods" like peanut butter regularly because I know that I cannot eat enough bulk at this point to do that with the leaner foods that had composed my previous diet.
With your weight and calorie intake (less than 800 calories), you probably should gain weight with the help of a physician or dietician. Your diet will probably involve eating foods that you are not comfortable eating on a regular basis and more than you are comfortable with, but you have to do it. Those health article saying losing weight is healthy are not for us. You are harming your health more than an obese person does. I know that it is hard, but if you want to have a future, you have to gain weight and eat enough calories.

I weighed in at 70 pounds yesterday. I measured my arms, waist, hips, ect... And have gained a full inch everywhere! I am upset-yes I want to gain 10 more pounds, but I am scared that now i'll keep gaining and gaining and by next year i'll be obese. I have to eat now every 2 hours or I get dizzy and this is really upsetting me. I felt like tearing my hair out I was so mad. I know I am not fat but I feel that way because I am gaining at the rate of 5 pounds a week. I feel sooo hopeless!!!

I know how you feel on that and im sure most women and men feel that same way. I myself and just finding it rather hard to lose weight and I know when I hit that weight that I will want to lose more. No one notices anything in me. They just say I look healthy. Well, of course I do. Im not eating and when I do I eat to much, but never throw up. No one knows what its like being a man or a woman over weight and feeling the need to look like brittney spears or lindsey lohan. I dont think most guys and vis versa realize what they are seeing is not real. But to someone with an eating disorder like myself and others on this board, we realize that they are not real but we want so much to become them and wanted by every single male and female that its to hard to resist.