Here you can find precompiled .hex files suitable for uploading straight to your keyboard's controller. See the instructions for uploading to get the files loaded onto the keyboard.
You'll need to determine which variant you need. If your board is newer than March 2016, (or if you flipped the board during construction at an earlier time) you probably have the standard build with the PCB labels facing up. If it's older but still has a PCB, you probably want pcbdown. If you have an older board with no PCB at all (hand-wired), it's probably using a teensy.
If you prefer not to use the web-based configuration, you can build from source if you don't mind installing the AVR compiler toolchain on your machine; this allows you to keep your layouts in a version control system.