1) What are the benefits of leaders? I can't find them in the rulebook...
Leaders are covered by Optional rule [16.9]. When using that rule, any attack made outside the "command span" of a friendly leader is subject to a -1 penalty. Basically, the basic effect of leaders is that they will tend to concentrate combat in specific sectors of the battlefield (you can still attack anywhere, but the efficiency of those attacks will be lessened if there isn't a leader nearby).
2) Is the dice-roll for crossing the pantoon bridge done when you move onto the "pantoon bridge marker" or afterwards?
The crossing roll is made when the unit moves onto the pontoon; if the unit fails to cross it gets moved back into the approach hex leading up to the bridge which often causes a big traffic jam.
3) Can you please give me a whole example of a combat situation? I don't know if I understand the mechanism correctly.
Combat occurs during the phasing player's Combat Phase; the phasing player is the attacker. If he has one of his units in an enemy ZOC (generally speaking, in a hex adjacent to an enemy unit) then he must attack that defending unit. To resolve the attack, compare the Combat Strengths of the Attacking unit(s) to the Defending unit(s) and, after applying modifiers for terrain, roll a die to determine the result. I recommend that you download the most recent (v4.1) version of the rules from the VPG website. There is an excellent example of a combat situation provided on page 12 (Ex.9.4.2).
Hope that helps.
Developer for VPG's "Napoleonic 20" series