Start disk 1 with the word BGM pressed down, and start disk 2 with GO pressed down.
Press during the game Shift and choose the last option but one.
While playing, press Shift to open the game menu. Select the 3rd option and then highlight the BGM option. Now, hold C and wait for a text screen to appear completely, once it appears and while still holding C, press Up, Up, Down, Down, Right, Left, Right, Left and X. If done right, the debug menu will appear.