How do you add radiation to a specific area? E.G. the geck chamber?

There should be under Statics or Activators, a Radiation marker. Don't know its ID off hand. But once placed, it will look like a radiation symbol, you double click it to set its area of effect and the rads amount.

Something to note, the rads you set will be how many rads per second the player receives when standing in the center of the radiation bubble.

Immunities to radiation will also affect how much radiation the player recieves. Perks and Armor can give radiation resistances.

Sry for the reviving this, but would you use this same method for making water radioactive? Such as you only get rads when touching the water and not whenever you walk into the radius of the radiation marker?

Radiation 'in' water is based on the properties you set for the water type.