PROTEUS ARES PCB double side Layout - Force bottom side lyout for selected devices
You can do this in 5 steps: (my version is 7.9 sp1)


Technology >> Design rule manager
a new window will appear >> Net classe tab >> net class >> power
pair 1 Hoz and Ver make them bottom Copper.
and for net class Signal also Hoz and Ver will be Bottom Copper.
make a suitable board Edge
Track mode (The line that you will use) Edit it as you want
Technology >> Layer usage >> uncheck all inner and mech layers(Don't use any of them)  
Technology >> Layer Pairs >> make all Bottom Copper