Hey!
Where do you get the pop-up?
In-game? Or is it a system message?
What does it say exactly? What's your version of the game?
The mod comes packaged in .zip archive, which you need to take it out of in order for the game to read it.
Check out this page, make sure you've followed all steps to the dot, especially Step 5. as that's what I guess might be wrong: https://wonderfulwhims.com/installation/