When added this mod to Minecraft, the screen is gray, but I can still move around

Needs more info. Maybe give this article a read.
Version?
Other mods?
Log?

Not a big chance...
Do you use optifine?

I have had this issue before. Nothing seemed to fix it. Then I came across a fix. Delete optionsof.txt. Relaunch the game. Disable Fast Render and Render Regions again. Now it should work.