You could unpack the grasses using Corsix... and you might have already seen this guide: https://www.moddb.com/mods/coheastern-front/tutorials/more-grass-types

1. But if you want it to be quick and easy... just download them from this link:
https://drive.google.com/open?id=1m00QRABQZAn9Th39q6lcp-JFkt7F18Eb

2. Extract the grasses folder into the following folder if you are using the original CoH:
C:\Users\Username\Documents\My Games\Company of Heroes\ww2\Data\Art\Scenarios

If you are using the New Steam Version for worldbuilder then the folder is:
C:\Users\Username\Documents\My Games\Company of Heroes Relaunch\ww2\Data\Art\Scenarios

It should look like this:


3. Now open Worldbuilder, click on the grass tool and press the little plus button.
Browse for the grasses folder you just created and choose 'types'

C:\Users\...\Documents\My Games\Company of Heroes\ww2\Data\Art\Scenarios\grass\types




Now your grasses should work. Happy map making!