Hi Tim. You're going to need an RCA socket like this one:
This will allow connection of a cable from your CD player onto the Audiophonics board. You can solder two wires from the Audiophonics board to the RCA socket. One is the signal and the other is ground. Your yellow board has four SPDIF inputs as shown in the second pic on your link. Solder two wires from the input labelled IN1 on the board to the RCA socket. I think the hole nearest to the 'IN1' white writing should be connected to the centre pin on the RCA socket. Then solder another wire from the top IN1 hole to the leg on the RCA socket. This will give you one SPDIF input onto the board. You could in theory add three more, but I am not sure how to switch between.
The I2S output is labelled I2S on the yellow Audiophonics board (to the left of the SPDIF inputs). On the DAC mainboard there are pins for connecting the I2S input. You need to connect LRCK, Data, BCK and GND from the Audiophonics board to the DAC mainboard (use the four pins labelled DAT, LR, BCK and GND), not the other I2S input right next to the one I've described on the mainboard.
If you don't want to make hard (soldered) connections between the Audiophonics board and DAC mainboard, you can buy pins to solder into the holes on the Audiophonics board, then get short cables that are male at one end and female at the other. That way you can easily disconnect the boards. You can get the pins and cables from eBay.
Someone correct me if I'm wrong, but I think you can only have one input on the red mainboard. It's either the WaveIO OR the Audophonics board, but not both. On the blue board Doede added SPDIF in addition to I2S which is switchable.
Do you mean the 5v supply that came with the Wave IO board? It depends on the current draw of the Wave IO plus the Audiophonics board added together. I can't remember how much current draw the 5v supply can provide.
PS Don't forget to completely disconnect the WaveIO board before you try the SPDIF board. Remove both ribbon connectors between the Wave IO and DAC mainboard.