Doing the following will apply the mod to your existing save -- I figured I would put the steps of the commentor's solution for the less technically inclined:

~ Go to folder of the mod -- you can do this quite easily in the launcher by going to
>> All Installed Mods'
>> Select the ellipsis on this mod (more options button)
>> Select 'Show in Folder'
>> Navigate through to common/on_action/balanced_buyable_building_slot_start.txt
>> Edit 'on_game_start' to 'on_perks_refunded'
>> Save file & launch/relaunch game -- after saving the .txt file, don't close it yet (convenience)

>> Select the 'Reset Perks' button in the lifestyle menu

>> The mod should work from there (you should be able to see the new icon in the county view)-- *save your game*.

>> Revert the change -- go back into .txt file and change 'on_perks_refunded' back to 'on_game_start.' Save the file.

>> Now the mod will activate for every new game you start -- the mod is already activated for your current save. This step is to ensure you do not have to reset your perks every game you start to get the mod working.

>> The original file should look something like this (allow for strange steam spacing):