I have a Mac and when I open Minecraft and log on it says Enter a admin username and password. How can I continue?
Which OS X version are you using?
Do you have Java installed? Which version?
Are you running the official Minecraft launcher or some third-party tool?
The official Minecraft launcher will never ask you for your Macs administrator credentials.
Most third-party launchers (at least those I know of) don't do so either.
You should only need the administrator login if you are installing Java, which is required for Minecraft or within the System Preferences if you have to change the Apps Permission (to allow Apps that are neither from the App Store or an identified developer).