I spoke to Gerald about the very same thing. He said that it's ok to insert/remove the plug into the bias jack while the amp is on....your speakers may not like it, but it shouldn't cause any harm to the amp.
I initially followed the directions in Cruz123's post, but since have had no problems just plugging/unplugging the test jack while the amp is on. I do have my multimeter already turned on and set to the correct range BEFORE I plug in the test leads. The suggested bias point for the Six Pacs is 220mA.