Can I use this software to test any signal? What's it's voltage limit?
You can test any signal that your hardware will support. In other words, the software is limited by the hardware. So, if you're using the audio I/O built into your Macbook, then that's the hardware that will determine your measurement bandwidth and voltage limits.
You certainly can analyze circuit boards with your Macbook, but you do so at your own risk. You need to take care to ensure that you are performing "safe" measurements. High voltages could damage the input and or output circuitry of your Macbook. Peak input voltages accepted (without significant distortion) by the line input on a Mac may be as high as 5 volts (~10 volts peak-to-peak), but you'll need to confirm that for your specific model. (Performing a voltage calibration, according to the tutorial in the SignalScope Pro Help documents, will tell you what your full-scale input voltage is.)
Feel free to post any further questions.