What is going on is not a direct spiral motion of the ions, but a statistical bias in the collective motions induced by the current and by the magnetic field acting in tandem. In free space the voltage applied along one axis would exert a force on a charged particle which would cause it to accelerate in the direction of the electric field (if it is a positively charged ion). The magnetic field, however, would deflect the motion into a circular motion around the magnetic field line. For an ion in solution, however, the ion will be jiggling around due to thermal motions; when exposed to an electric field it will pick up an extra velocity component adding to its motion along the direction of the field, but it will smash into a solute molecule in about 0.01 nanoseconds and bounce off in a random direction. The extra component of velocity is very small compared to the thermal motions, but it creates a bias in the collisions that will add up into a small drift of the fluid in the direction of the field. Calculating how much drift looks to me to be fairly complicated. (If you are interested I can point out some of the considerations needed to make this calculation.)
You say you are using a bar magnet. I don't think that will be optimal to produce a uniform magnetic field that points in one direction over an appreciable test volume. Also, you must use one of these (relatively) new neodymium magnets in order to get a magnetic field large enough to see anything. The demo probably had a small, 1 inch neodymium disc magnet under the cup. The ideal configuration for very high fields would, I think, be two axially polarized ring magnets with a central bore of about 1 inch placed one above the other with a separation of about 2 inches to give you a small (~1 cubic inch) working area and a VERY large field. Here is a reference to the kind of magnets I am thinking ofhttp://www.kjmagnetics.com/proddetail.asp?prod=RY0X04
I'd guess you'd get a field of ~2 T [my eyes pop out in amazement]. You will need a very sturdy frame to hold the magnets, made of wood or aluminum or some other strong, non-ferrous material. Be sure to epoxy the magnets into place so they can't escape their supports.
The fluid used should be as conductive as possible. For salt water, be sure to get as much salt as possible into, perhaps by boiling the water and a pile of salt briefly, then filtering out the remaining salt in the cooled solution.
********If you start to work with powerful neodymium magnets be extremely careful
of your fingers, they can literally be cut off or smashed to pulp by magnets like these snapping together (the magnet rig mentioned above would exert a force of a hundred pounds or so between the magnets and much more as the distance between the magnets decreases). Also, wear eye protection, since these magnets are brittle and if something snaps onto one chips could fly at high velocity. Adult supervision IS MANDATORY.***********
Looks like mud on my face! I seem to be way out of date on what kind of magnetic fields these neodymium magnets produce. When I studied EM theory, an 0.6 tesla magnet, as used in one of the videos that actually gave numbers for the relevant magnetic fields and currents, needed a magnet as big as a room!! (I've attached a picture.) With this kind of field you can indeed get a little movement.