Here is the link to the instructions for the Splitter.
I am not sure about the Add-On for Labs. If I were facing this, I would try to uninstall the Tiller Labs Add-on, (Add Ons > Manage Add Ons > Tiller Money Labs > Uninstall.) Sign out of your Google account, close Sheets and your browser, and then, after re-opening your Tiller Sheet, (you’ll need to log back into your Google account) try to install the Labs Add-on. (Add Ons > Get Add Ons > Find Tiller Money Labs > Install) (I tested these steps on my sheet just now to confirm you won’t loose anything… )
Note, too, that after the Labs add-on is reinstalled, you can access the Splitter in the newly expanded menu options (Select the row with the transaction to be split > Add Ons > Tiller Money Labs > Transactions > Split Transactions)
Way over my head here on the technical side. Others may be able to help more.