Did you disable “safe mode”? It is needed so that the RPi boots up: Excerpt from the RetroPie GPIO article:
Adding a jumper between pins 5 and 6 of P1 results in /boot/config.txt being ignored (except for avoid_safe_mode) and a default cmdline.txt is applied, followed by loading kernel_emergency.img. As stated in the official forum
if you connect external hardware to that pin, the worst that will happen is it falsely triggers safe mode.
To avoid this safe mode when the adapter is attached a setting has to be made in /boot/config.txt. This could be done, for example, by opening the config.txt with
sudo nano /boot/config.txt
and saving the changes with “Ctrl-X”, which has to be confirmed with “Y”.
Also, the 2×5 pin header needs to point inwards, i.e, the center of the RPi PCB.
Could you post one or two pictures of your assembled adapter here?