Following the instructions, when I search for "pico" in the Boards Manager the only two options that appear are "Arduino Mbed OS RP2040 Boards", which I have already installed version 4.0.4 from my previous attempts, and "DEPRECATED - Arduino Mbed OS Boards". I do not see Earle's "Raspberry Pi Pico/RP2040" package at all.
What the hell am I doing wrong? I have Arduino 1.8.19 installed, that should work fine. Could my employers firewall be part of the problem? I can get Admin rights I just have to request them for short periods of time.
Hold the phone, I might be onto something with my firewall question. I removed the other two URL's and uninstalled the Arduino board packages I had installed this morning, leaving only Earle's URL. Now I get this just opening the IDE software.
I think the firewall is probably the cause. Arduino IDE must be able to download things from the Internet in order to populate the Boards Manager dialog with the 3rd party platforms, and will also need it in order to install the platforms.
Arduino IDE only absolutely needs this Internet access for the time it takes to make the installation (though you would also need it later if you wanted to update to new versions of the platform), so even if you could get just a temporary exemption from the firewall restrictions that would be sufficient.