Try this...
1. Go to "Lights" in the Object Window
2. Find a suitable Fire effect, and move it into position.
3. Double-click on it, and in the top box give it a Unique ID
4. Create a new Activator
5. Use this Script:
Short FireStateBegin OnActivate If ( FireState == 1 ) ; On Set FireState to 0 [FireRef].Disable ElseIf ( FireState == 0 ) ; Off Set FireState to 1 [FireRef].Enable EndIfEnd
6. Attach that to the Activator
7. Place the Activator
That should do it-- I'd suggest using either an invisible collisionbox activator, or the actual wood for the fire as the controler