Ok I think we can get you up and running again pretty quickly. You have to drag the Sia-UI application to my Applications folder manually. That way you can pull it up as normal. When you fire up the Sia-UI app it will automatically create all necessary files under your ~/Library/Application Support/Sia-UI directory.
So to get back up and running:
Exit Sia-UI, Use Activity Monitor to make sure Sia-UI really quits, that is always a fun thing to debug.
Stash anything under your your ~/Library/Application Support/Sia-UI and restart the Sia-UI.
Wait for your Sia to fully synchronize the blockchain. This will take a while...
When you relaunch it, go to the wallet plugin.
It should recognize you don't have a wallet present and ask you to make a new one.
Do so and record the new seed.
Click the "Recover Seed" button and put in your old seed.
Wait for a blockchain re-scan and you should be good to go!