Il Mio Problema
The right button on the D-pad (cruciform shaped button) on my sons DSi was not working which prevented him from using it to plays games, etc. The actually switch buttons for the D-pad are located on the power supply board. They are little 'dome' switches, one for each direction.
La Mia Riparazione
With the disassembly and repair info at ifixit the repair went very smoothly. The power supply board is one of the easiest boards to remove from the DSi. With my bad eyes I needed a magnifying glass to help me open/close some of the tiny connectors but it really was not too bad at all. You need to transfer one flex circuit and the attached LED assembly from the old board to the new (which is shown in the ifixit instructions). The entire repair took me about 30 minutes as I took my time so not to break anything else. FYI it took only two days for ifixit to ship the board from California to Massachusetts. That is what I call great shipping! My son is VERY happy now.
Our DSi stop working (insert son's unhappy face here). The charge indicator lamp (the middle lamp) was flashing and the unit would not power up. I decided to open the unit up again and I noticed that the two tiny wires (red and black) from the new PS board I installed were 'mashed' as they had gotten pinched by the plastic case during reassembly. I can see (using a magnifying glass) a bit of the wire conductors where the insulation was flattened on both wires. I did not notice the routing of these wires when I removed the old board but now comparing the old (original) board and the replacement board I notice that the wires on the replacement board are a bit longer and therefore the extra length must be placed carefully to not pinch them. Upon further checking however I have determined that although the wire conductors are still intact they must have shorted out because the tiny SMT fuse 'F1' on the new PS board is blown. I will have to chalk this up the 'installer error' and order another PS board ($22.45). I will update when that board arrives.
Il Mio Consiglio
Study the disassembly instructions carefully and take your time. A #2 micro JIS/Phillips screwdriver will be essential. The two little rubber 'feet' that cover 2 screws fall apart in trying to pry them out but you really dont need them anyway. I did not have a plastic pry tool and really didnt need it to get the main case open but I can see where it would be nice to have.
Update: Be VERY careful in the placement of the tiny red and black wires that go from the PS board to the main board so that they do not get pinched between the case halves as the replacement board's wires are longer so there is more 'excess' to get pinched.