To get more energy you have a few options. You can simply wait and allow it to regenerate, which is the easiest way, but takes time.
You can also request energy from your friends, which is probably the best way to get it quickly - if you have an extensive network of people to ask. You will receive 5 energy points from each. Friends can also gift you 1 energy point each day - it is a good idea to return this favor to them and you should have energy coming in from all over the place.
Your energy bar fills up whenever you go up a level. This is a great reward and if you plan it correctly, you can use up all your energy just as you're about to go through a level and so get it all back again. Also, it can be worth buying a building or decor item just to give you the XP to go over the edge.
Finally, you can buy energy using gold, which is instant but expensive!
A lot of gamers have had problems unlocking the Dragon Tower via the Dragon Snare quests.
To unlock it, you need to get four stars on one of the chapter three scenes. The difficulty is that the scene required is different for every player. The best thing is to work your way through them and it should unlock.
Some users have reported the quest not opening up even after fulfilling the above, in which case you should contact Playdom and raise a support ticket.