You can also read the JQ set up guide, you can find it here;
http://jq-products.com/the-news/43-t.../160-the-guide
Although aimed at 1/8th buggies as is the Hudy set up guide the principles are the same.
My basic understanding of 'pack' is that we're trying to make the suspension to firm up when it has to move quickly like from landing after a big jump to stop the car bottoming out but when the suspension is moving more slowly like when driving over small bumps in the track it is allowed to move more freely.
Read the JQ set up guide and just skip the nitro specific parts, I find that it has some really good tips and pointers for finding a basic set up.