I tried to install the SDK from the aforementioned website and couldn't find a trace of it once the installer finished. Though I did find some libusb items at /usr/local/Developer/SDKs/MacOSX10.6.sdk/usr/local/ I'm not sure it came from that installer as I couldn't find any examples, just libraries and they were labeled as libusb0.1.4.
On the other hand, since you asked your question with the macports tag, I'd suggest you use that to install libusb as it at least puts everything in a known location:
- /opt/local/var/macports/software for install files
- /opt/local/lib for libraries
- /opt/local/include for header files
If you've never used macports then once you have macports installed then you can install libusb using
sudo port install libusb
to install libusb1.0 or
sudo port install libusb-legacy
to install libusb0.1.12.
Then you can create a project in Xcode and link to the libusb-*.a library contained in /opt/local/lib (in my directory my library file is called libusb-1.0.a since I have libusb1.0 installed).
I don't have much experience using Xcode so I can't help you any further down this path. Personally what I've done so far is use a Python wrapper called pyusb to do some quick experimentation with libusb as I don't have to do IDE setup or library/include file setup with Python.
UPDATE: Poking around the macports install directory for libusb, I didn't find any examples included which was a bummer. If you've gotten this far then I'd suggest going directly to the source for libusb, download and unpack the tar file, and building the code that's in the example directory. Sorry for the roundabout way of getting libusb examples but I've really come to appreciate the organization macports imposes on installations which saves me quite a bit of future pain (like when I want to uninstall).