Once you have entered the table, use your left mouse button to select the value of the chip you wish to bet in the lower left corner.

When you have selected the chip, use your left mouse button to place the chip on the table by clicking in the "Player", "Banker" or "Tie" (above "Banker") field. You can place multiple chips by clicking mutiple times. You can regret a wagered chip before the game begins by clicking on the wagered chip with the right mouse button to remove it.

When you are finished, simply click "Done" in the lower left corner (above the chips), and the game will start.