The bolt that runs through the brake has a couple of flats on it just in front of where it meets the front fork. You can put a spanner (of the thin variety) onto these and align the brake.
That is the proper way to do it, but this is not always practical. The other approach is to tighten the allen key bolt at the back of the fork and then undo it by a small amount, the small amount required to lift the left-hand-pad (as seen from the front) off the rim.
Before doing any of the above it is worth considering the wheel - is it in the fork properly? The best way to ensure the wheel is correctly in place is to put the bike on the ground with your weight on the handlebars. Now push the quick-release skewer tight.
If you do not have quick release on your front wheel then you can tighten up the nuts on each side bit by bit, again with weight on handlebars so that it does not end up being to one side.