Installing on macOS
Although StepMania 5.1 is currently in beta, it offers many great new features and should already be stable enough to replace older versions of SM5. You can read more about SM5.1's new features in the release notes for beta1 and beta2.
Unfortunately for macOS users, StepMania's macOS code hasn't kept up with the times, and installing any version of SM5 on a modern version of macOS is currently tricky. This page will describe the steps necessary to install and run SM5.1 on macOS.
Start by downloading the macOS version of StepMania 5.1 beta from its GitHub page.
After this step, you should have the .dmg file in your Downloads folder.
Double-click the dmg you downloaded to mount it in your filesystem. The window that automatically opens should show a single folder titled StepMania-5.1.0. Drag this folder to your Desktop.
Open the new folder on your Desktop, and double-click on the StepMania application to start it. You will likely be notified that "StepMania" can't be opened because it is from an unidentified developer. Click OK.
From the Apple Menu, go to System Preferences... and choose Security & Privacy from the first row. Click Open Anyway to grant SM5.1 permission to open.
Double-click the StepMania application on your Desktop to attempt to start it again. If it starts to load songs, you can stop reading now and enjoy SM5.1 beta.
If you are given an error message like Metric "Common::ScreenWidth" is missing. or No NoteSkins found. click Abort.
You've encountered a known bug that can be resolved by running a command from your Terminal.
For this step, you'll need to use your macOS Terminal. You can find it by pressing ⌘ + Spacebar to bring up Spotlight Search, typing Terminal, and pressing Return.
Assuming your StepMania-5.1.0 folder is on your Desktop, copy and paste this command into your terminal, and press Return to run it. This command will not work as-is if the folder is not on your Desktop.
xattr -dr com.apple.quarantine ~/Desktop/StepMania-5.1.0/Assuming it worked, the Terminal will not give any error messages or really do anything. No news is good news in Terminal-land.
You should now be able to close the Terminal window and successfully start SM5.1 beta.