White streets usually originate from missing textures.
The readme of the Addon describes how you can resolve this issue.
I pasted the corresponding instructions down below:
1) copy "Splines/Marcel/texture" folder to "Splines/ADDON_SimpleStreets/texture"
2) copy "Sceneryobjects/Kreuz_MC/texture" folder to "Sceneryobjects/ADDON_SimpleStreets/texture"
3) copy and RENAME "Sceneryobjects\Railroad_RUE\texture\Gleis_Betonschwellen.bmp" TO "Sceneryobjects\ADDON_SimpleStreets\texture\Gleis_Beton.bmp"