Slic3r controls the fan in 'Cooling' section of the 'Filament setting' tab. I don't use Replicator G, but I would have thought is supported fan control. Check your fan works from the command line first, by trying the following from [
reprap.org]
M106: Fan On
Example: M106 S127
Turn on the cooling fan at half speed. Mandatory parameter 'S' declares the PWM value (0-255). M106 S0 turns the fan off.
M107: Fan Off
Deprecated. Use M106 S0 instead.