Admin page won't load

Hello, please forgive me for what may be a simple problem. I've installed the admin plugin according to the instructions, but upon pointing the browser to my localhost for the first time, I'm getting "URL Not Found: localhost/admin" error. I'm not sure if this is a problem with the installation or something on my local server side. I'm unfortunately very inexperienced with this and just fumbling along. I'm working on a Mac with local web hosting turned on, to use my machine as a test server. Any help would be much appreciated.

Try this Page Not Found help