Mattoco: Correct, script modifications are done through Bioware's Aurora Toolset, which should be accessible from the launcher or by opening nwtoolset.exe in your NWN directory. Once you load the Random Dungeon Generator module, you can either open the script editor from the Tools drop-down menu, or find the script you're looking for on the left side of the window as mentioned and open the script editor in that way.

Note: If you have "Automatically Compile Scripts on Save" disabled in your Toolset Options (usually done for larger modules to prevent the toolset from throwing errors), the following is applicable, otherwise everything will be handled when you save changes to your scripts:
After you save and make changes to a script, you sometimes need to compile the scripts in your module for the changes to take effect (this process generates a computer-friendly version of the script that the game reads). This can be done through the Build Module dialog box under the Build drop-down menu. Choose to compile scripts, build, and then save your module to lock in the changes.

If you have further scripting questions, the official forums are a good resource for this as there are many builders and scripters who frequent them that have been working with the game for years: Bioware Social Network - NwN 1 Builders - Scripting