First of all...not all amps have a bias adjustment. Some have a fixed bias, some require a little surgery...again..I'm not the expert so I'll let them clarify this.
The instructions that came with the probe are here:
http://www.webervst.com/biasread.htm
On my amps, there's a little hole next to the transformer that has a recessed trim pot screw. I first plugged the probe into my multimeter (not necessary if you buy the one with the readout). I plugged the tube into the probe, plugged the probe into the amp...turned the amp on and let it warm up. I adjusted the bias trim screw until the multimeter read 35 millivolts. (your amp may have a slightly different recommended setting)
Then...turned everything off...switch to the other tube and checked if it was near that value. I think the idea is to get them in the right range (around 35 millivolts for my amp). You want to pick a setting that keeps them both in the right range. If one tube reads a very different value than the other, you probably want to get a better matched set of tubes.
Takes about 10 minutes and saves you a trip to the amp tech for tube changes.
I'm sure a real amp expert (ala Cookie) will be able to give more accurate instructions.