Once you've enabled SCG in Blender, it creates a panel accessible in the toolshelf of the Blender 3d viewport. To see it, put your mouse in the 3d view, then press T to display the toolshelf. Scroll down, and you should see SCG's options.
This section explains what these controls are for.
This lets you set the maximum amount of memory (RAM) you allow SCG to use on your system. On 32 bits systems, values above 1200 might not work. SCG might not even start. If this happens, set this to 1200 or below. On 64 bits systems, you should be able to go much higher, up to several gigabytes, but it varies from system to system, so experiment.
Do not increase before you know SCG needs more RAM. You'll be able to see how much RAM SCG is using during the generation. Increasing this value while SCG doesn't need more is useless.
This option is necessary due to the way Java works. SCG runs on Java, and with this slider you're telling Java how much memory it can use at most. If SCG ever needs more than that, Java will probably crash. This won't do any harm to your computer, but you won't be able to make bigger cities. In that case, increase this slider. If there is any memory-related problem, you'll have this error.
Click to delete all objects having a name starting with SCG_. Lots of objects get created by SCG, and you may not need them anymore. Delete them with this button, without touching the rest of your objects.
Opens the full set of options to control the aspect of your cities. Options are organized in tabs. Don't forget to save before closing. The next sections of this documentation explain in detail what these options are.
Launch the generation process, taking into account all of your settings. You can see the progress, the amount of RAM used, and any message SCG may have to communicate. If SCG crashes because it doesn't have enough RAM, then use the max RAM slider.
You now have a 3D city. If you create another one, SCG will overwrite the current one. If you want to make other cities, and keep this one, remove the SCG_ part in the city and terrain names, and they won't be overwritten.
This happens when SCG does not output anything, then Blender complains about that.
The main reasons why SCG does not output anything are:
Finally, you can have an idea of what your city will look like in the final render by clicking this button. This may put some strain on your video card, especially with large textures, because SCG's materials are complex for realtime display. If Blender crashes, reduce the textures size, or do not use this button.