The .dll error is most likely due to you not running the tools from the Sins installation folder. They use shared resources, and that's where the dlls are.

But the core of the modding is done with trusty old Notepad (or Notepad++, much better). There's a very nice intro/tutorial on that in the sins_modset.pdf in that Documentation folder Zero10oneSB mentioned.

The programs included in the tools are a map maker, a particle maker (weapon effects, ability effects), and a developer game executable that lets you spawn units and such after you launch the game, for easier mod testing.