Making pin assignments
Go to the "Assignments" menu and select "Import Assignments...". Import the de0-nano/rgbmatrix-fpga.qsf file. After you do this, a message should appear in the "System" console tab at the bottom of Quartus: "Import completed. 14 assignments were written (of of 14 read)."
You can (optionally) customize the pin assignments that were imported by going to the "Assignments" menu and selecting "Assignment Editor". Additional information on the GPIO headers can be found in the DE0-Nano PDF manual
(pages 18-20). A mapping of FPGA pins to GPIO headers can also be found in the de0-nano/DE0_Nano.qsf file (open it with a text editor).
Save any changes. Now we are ready to connect the pins on the FPGA to the pins on the RGB LED matrix panel!