I've opted into the beta by following the instructions, but the Install button isn't shown, and if I attempt to install Terraria using the contextual menu, then the following error message is presented:
Terrraria is not available on your current platform.
Restarting Steam had no effect.
Edit 1: Enabling and then disabling offline mode (from the Account submenu) resolved the problem. Yay!
Edit 2: When I now attempt to launch Terraria, the following error message is presented:
Failed to start game (missing executable).
I opened Terraria's installation folder and discovered that it's completely empty!
Edit 3: After opting into the beta again, I can now play! However, there is no background music whatsoever.