Calibre Companion beginner

I've just acquired a Nexus 7, and am searching for ways to be able to read my ebooks on it - not those I've bought through Google Play, the ones I have in my calibre library.

I have bought and installed Calibre Companion, but haven't been able to make a Wi-Fi connection between the Nexus and my calibre library on my Windows 7 computer. So:
- Is there anything like a 'Calibre Companion for Dummies' anywhere so I can understand what I'm doing?
- Is there any way to use a USB cable for the connection? That's what I do for my ereaders; I don't quite see the point of using Wi-Fi for the Nexus.

I'll probably have lots more dumb questions, so thanks in advance for your patience.