Do you own or have access to a multimeter? You need to measure the voltage and current going through those switches. If they are TTL (or TTL compatible), you might be able to drive them directly from the arduino pins.
Unless they are pushing a lot of power through those switches, you probably don't need an opto-isolator. A simple transistor will probably work just fine. This tutorial http://itp.nyu.edu/physcomp/Tutorials/HighCurrentLoads will probably tell you what you need. The transistor is the only critical part.
Um, one question: why don't you just wire the switches closed? Do you actually need them?