Please read the details!?

I'm of 22 years age, 5'7", 56 kg ... am I underweight? also, I am into regular intense physical activity [read playing football regularly], a few months back I wanted to join a gym to tone up my body.but the fitness instructor there suggested me to gain a few kilos first, is it true that you can't do physical exercises if you are underweight? also he suggested me a diet [which to be frank was confined to the suggestion of taking meals at right times] but after two months, I can hardly say that his suggestion worked!!! how can I gain a few kilos? 10 points to the one who answers all these three questions correctly and effectively x)