When is it possible to buy an add-on or rebuy?

At the start of the Tournament, each player will receive an equal amount of Tournament chips. 

Note: starting chip stacks may differ per tournament

During the Tournament, every player has the opportunity to rebuy  during the first stages of the event - usually for the first 1-2 hours, with a break when the rebuy period is over for players to buy a final add-on to their stack before the Tournament changes into a normal Freezeout Tournament type.