The path will be something like:
/Users/<USERNAME>/Library/Group Containers/<RANDOM_ID>.com.realmacsoftware.rapidweaver/Library/Application Support/Addons/Elements/
Not sure how random the RANDOM_ID is but it might start with P97H...
The folder should contain the com.weaverpixel.flexon-v2-b4.elementspack as the latest version and not the earlier one.
There maybe an easier way to get to it but that is where I found it after searching for *.elementspack
Hope that helps. 
