Hey there!
It sounds like you're facing a couple of challenges with getting Minecraft set up on your older iMac. Since your Mac is running High Sierra and it’s a 2011 model, you might be limited in terms of the versions of Minecraft that can run smoothly on it.
Firstly, it's totally normal that you needed to create an Xbox account since Minecraft uses that for login purposes, especially after Microsoft's acquisition of Mojang.
For downloading the right version of Minecraft, you'll want to focus on the Java Edition, as it's more compatible with older systems. Here's what you can do:
- Visit the official Minecraft website.
- Navigate to the "Games" section at the top of the page, click on "Minecraft".
- Choose “Get Minecraft” and then select “Computer” from the list of available platforms.
- Click on “Mac” and it should give you the option to buy/download the Java Edition.
Once you download the installer, open it and follow the instructions to install Minecraft. After installation, open the Minecraft Launcher. The first time you run it, you might have to log in with your Xbox account that you created.