Multi-Claude is signed and notarized by Apple, so it installs like any other Mac app.
Open the file and drag Multi-Claude into your Applications folder.
Double-click Multi-Claude in Applications. The first time, macOS asks you to confirm that you want to open an app downloaded from the Internet. Click Open and you're in.
Paste the key from your purchase email, or click the activation link Lemon Squeezy sends you and Multi-Claude will activate itself.
On first run, Multi-Claude offers to import your existing Claude install as a named profile (e.g. "Personal"). After that, click + New to add a second account.