Multi-Claude is in beta and ad-hoc signed (not yet through Apple's Notary Service), so macOS asks for confirmation the first time you open it.
Open the file and drag Multi-Claude into your Applications folder.
You'll see a dialog warning that Apple cannot verify the developer. Click Open. macOS remembers this — every future launch is a normal double-click.
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.