I've had a look and you don't appear to get mirroring in the free version, so no option of live mirroring.

Your best workaround is probably just to do your half sketch and then when you want to mirror it: draw a vertical line about where you want to mirror, go into the 3d view, press move, click on the surface/lines that's been created, move the "move cursor" onto the line and rotate it over 180 degrees.

Hopefully that makes sense, but if not I'll try to explain again