Okay. You open regedit. Then you go to HKEY_LOCAL_MACHINE/Software/Volition/FreeSpace
through the tree system on the left. Then you'll have a window with a few values on the right.
In the right window, right click into empty space. Find DWORD value underneath 'new' and click it.
You'll get a new entry, and it'll be called 'New Value #1'. Replace that with D3DTextureDivider.
Now right-click the D3DTextureDivider entry and pick Modify. You'll get this...
Change the value to 1 (or whatever the guide indicates). Then you'll get this.
The registry entry is now intact and you're done