ANSWER

Once you have placed a cell that you wish to make spawning, critical, or no-bomb, simply right-click on it with the mouse. This will bring up the Cell Properties screen, where you can configure the cell in a number of ways.

Spawn balls from this cell

Check this option if you want to have balls placed on to the playing field at this cell. You can then set additional spawning properties.

Critical cell

Check this option to designate the cell as critical.

No-bombs cell

Check this option and bombs will be vaporized when they enter the cell.