Updated rules are available on the offical Heroscape site. There is also a rules FAQ page. If you have questions that are not answered in the books that came with the game check their site. There are some added rules that were not printed.
Gameplay in a nutshell
Heroscape gameplay is fairly simple and straight forward. The card for your figure shows how many spaces you can move, how far you can shoot, how many dice you roll with for attack and how many dice you roll with when you defend.Before each round you place markers on the card of the figures you are playing with. This will be the order in which you move your figures. You cannot change these markers once the round has started, so there is a bit of strategy in this aspect of the game.
After everyone has set their turn markers you roll to see who will go first for the round (three turns). Now you take turns (usually clockwise) around the table. You will flop down your number one marker for your first turn and move your unit. If you can, you will want to attack or at least gain some high ground. The dice in this game are "loaded" for attack.
Having height advantage is very important. If you are higher then the unit you are attacking you will get an extra attack dice. If you are defending and you are higher then the attacking unit you will get an extra defensive dice.
Each figure has at least one special ability and most of them are explained fairly well. There are also a few simple movement and line of sight rules.
That about sums it up. Once you have played a couple games you will find that the game can move quite quickly.