A shot like this does take a bit of setting up to get right, here is what you'll want to do:
For your scene, more important than it being a dark room is for the background to be dark, a light background will wash out the the object in motion. See here for an example of what I mean:
http://www.flickr.com …in/set-72157605284146844/
In the upper dark area the skater comes out much clearer than when he is over the light box.
Also remember that your background will be exposed multiple times, so even though it may look dark in a single frame, it may build up and become too light for the final image.
Before the ball gets rolling, set it in the middle of the frame and use a constant light source to be able to focus on the ball, then throw the focus in manual and leave it alone, you could even gaff the focus ring if you really want to make sure it doesn't go anywhere.
With the ball still at rest find your proper exposure with the strobe, you'll want your strobe to be no more powerful than 1/4 or so in order to allow you to give you enough pops during your actual shot before it has to recharge. For this example lets say f/5.6 @ 1/16 flash power.
Roll the ball across the scene and determine on average how long it travels in front of the frame. Also determine how many times you want to expose the ball. For our example we'll say 2s and 10 times.
Now switch your strobe over to multi mode. For this example set the strobe to 1/16 power, note you can go a touch less if you want more of a ghosting effect.
The additional options for multi mode are # of total flashes and frequency in Hz. A Hz is number of cycles per second, in this case number of flashes per second, so 1Hz is one flash ever second, 2Hz is 2 flashes per second.
For our example we want 10 flashes, so set # of flashes to 10. In order to calculate the Hz we take the number of flashes we want (10) and divide it by the number of seconds the ball will be in front of the camera (2), 10/2=5Hz (5 flashes per second).
Note that the strobe will not allow you to choose power, #, and Hz combos it is unable to do. If this is the case lower one of the settings, generally you'll want to lower power and adjust your camera's aperture or ISO to compassionate.
This should give you a good starting place for your shot, some additional tweaking will probably be necessary. Here are a few of the basic tweaks: (assumes ball is rolling from left to right)
No ball on left side: strobe/shutter firing too late, trigger it sooner.
No ball on right side: strobe/shutter firing too early or strobe not firing enough times, increase the number of times the strobe fires.
Too much separation between each exposed ball: frequency too low, use higher Hz.
Each exposed ball overlaps too much: frequency too high, use lower Hz.
Balls not 'solid' enough: increase exposure.
Hope this helps.
