If it is a pre-built library, you can simply drag it into your Xcode project (or use Project=> Add to Project…) in the same way as for the source files / headers.
If it has not been previously created, you will need to create it for any environments and architecture that you want to target. If it comes with an Xcode project, it's easy. If this is a regular open source distribution type, you usually do something like this:
$ ./configure
$ ./make
$ sudo ./make install
This usually puts the built-in library and header (s) somewhere like /usr/local/liband /usr/local/include. In the main Xcode project, you can simply add these headers and libraries to your project.
Note that if you are doing cross-compilation, for example. for iPhone, you need to add a few flags to the team ./configureso that you aim for the right architecture, for example. ./configure -build=arm-apple-darwin9.0.0d1.
, MacPorts, , Mac OS X - .
libical iPhone.