it should be on your hotbar. if you cant find it, press the windows key and search "file explorer."

after clicking it, copypaste this into it.
C:\Program Files (x86)\Steam\steamapps\workshop\content\1206560
(note: this does not work on MAC because everybody knows that aliens are the only ones who use it)
go through the folders until you find the mod you want. you can tell which mod it is by clicking the "icon" file inside of the folder. this will show you what mod it is.
(this is not a race mod, i am using this for an example real quick)

after locating your race mod, click the "+" button on the top. this creates a new tab.
on this new tab, copypaste this in the hotbar;
C:\Program Files (x86)\Steam\steamapps\common\worldbox\Mods

drag n drop the mod from your first tab into the second.
In the end, it should look like this;

this is the end. no need to give me rewards or anything.