First, some of what you have posted does not seem to be related to what you want to do. Specifically, the schematic you posted is for a tachometer. It uses reflected IR to generate pulses that are counted with the microcontroller (16F84) and displayed on an LCD as RPM or some other function, such as cutting speed for a lathe. Your project seems much simpler.
Refer to a redrawn portion from the posted schematic:
**broken link removed**
The +5V and ground come from the mouse. R1 limits the current to the IR emitting diode (IRED) in the sensor device. As the reflected IR light hits the detector, it allows more current to pass. Voltage drop across R2 increases, and thus the signal goes from +5V (no current through R2) to some much lower value. NB: The part I picked for the schematic is an optocoupler. In your device, the IRED is not pointed directly at the detector, but rather is pointed perpendicular to it, so the detector sees only reflected light. In your picture, the sensor is the rectangular black device with 4 pins.
The rest of the schematic you posted is not related to your project. I believe the mouse modification you show in your earlier post simply takes that "low" signal and treats is as a "click", i.e., switch closure, for one of the mouse buttons. At least, that's what the picture of the modified mouse PCB looks like has been done. You may need some interfacing, but it may also just work as is. You also show a small interface box. It is hard to know what is in it from the picture.
The "mouse click" from the optical sensor then keys the camera via a program in your PC. R2 is probably a fairly high value, maybe 1K to 10K ohms. R1 is probably just 100 to 200 ohm. I could not find the part number you reference in the source of datasheets I use.