Cannot locate mage.php

I’m convinced I am putting in the right mage.php path, but am consistently getting the error of “invalid url”.

So, I went back to basics and read the documentation on Installation, which starts out with my having to change something in the functions.php file, but the instructions don’t make complete sense to me, because the path ~/your-magento/app/code/local/Mage/Core/functions.php doesn’t exist in my installation of Magento. I put the copied and amended file into ~/your-magento/app/locale…but this didn’t solve my problem.

Basically I can’t even get out of the gate and I’m really struggling!

Thanks in advance for help…