Quote: "how can i open one of the mods and edit the source code"
It is very doubtful you will ever be able to do this. I, for one, would not give you my source code, so you wouldn't be able to open it.
Quote: "well, then how do people make these engine mods"
First, you need to purchase DBPro. Then you need to download Lee's source code. It is found in the "Extras" file for DBPro. Then you can open his source code and edit all you want. However, there is no guide. He will not be able to help you figure out his code. You have to do that yourself.
The code is between 45,000 and 50+ thousand lines long. I can't remember exactly how many lines. It took me a year to write and finish my mod. My mod has taken the file to almost 70,000 lines.