First off, you don't want to use the source from the 6.6 Extras. That is old code and not valid if you are planning to mod the current source (which is at v1.20 and the source in 6.6 Extra's is v1.07).
When you install DBP and your extras (such as Enhancements, DarkAI, etc), and you want to mod the FPSC source, you need to follow the instructions as outlined in the readme file.
Quote: "
How To Compile The FPSC Source Code
-----------------------------------
2. In your 'Dark Basic Pro' folder, inside Compiler folder,
rename the PLUGINS-LICENSED to XPLUGINS-LICENSED and then
rename PLUGINS-USER to XPLUGINS-USER to protect your current
installation versions. FPSC requires slightly modified DLLs
in these folders to compile the source code properly."
Then what you do is copy the "plugins-licensed" folder from the Extra's zip to your DBP "plugins-licensed" AFTER you have renamed your original "plugins-licensed" and "plugins-user" folders as outlined above.
I have "enhancements.dll", but it is in my "xplugins-licensed" folder. The
7.5 Extras has an "enhancementsFREE.dll", which is in my "plugins-licensed" folder.
You will need to ensure you are running an updated DBP, such as U77 so that your FPSC-Game.exe will compile and run on Win 7. Then you need to have the source of FPSC you want to work with, which is
here.
If you are modding v1.20 and are having issues getting your compiler configured, the best people to ask are Scene Commander, s4real, Ched80, or Flatlander as they work in the current source all the time. I am modding v1.17 so my compiler is set up for that source. I also used the Extras 7.5 to get the compiler files I needed, then grabbed the v1.17 source from the source page I linked above.

There's no problem that can't be solved without applying a little scripting.