Hi there. I'm trying to gain weight too. I have been told by a nutritionist to eat more things that have good fats in them, such as advocados. If you eat meat, she suggested salmon.
I have been taking a protein powder that I mix in with my soy smoothie and fibre. It's 100% safe and should cause some weight gain.
I also try to have Tofu cutie "ice cream" treats and make that AD brownie recipe often (modified since I don't eat eggs).
The main thing the nutritionist told me is to increase my calories. I think she said I'd need an extra 500 A DAY to start gaining a 1lb or 2 a week. It's hard to do, obviously, as we can't eat pizza or junk food that's loaded with calories. I try to snack on Luna bars and other things that are a tad higher in calories. I also make sure that my drinks have calories in them and aren't just water. I now bring a large container of chocolate soy milk to work with me every day. (I actually spilled it all over my desk on Friday though! - Oops!).