Hi
I've designed a few boards with DSPCB.
I'm using version 3 on Windows 7 Pro...awesome free PCB design tool.
Anyway...
Open the PCB file, click on Output from the menu bar, then click on "Manufacturing Plots".
On the right side, in the "Plots" pane, you'll see a list of plot output "layers" with check boxes next to each layer.
Just check or uncheck the ones you want generated.
Some options...
1. You can also just ask the fabricator not to use a solder mask on the bottom.
2. Any decent fabricator will usually know whether to use a solder mask or not.