As long as you stick with the Apple-supplied frameworks that reside in System/Library/Frameworks (Cocoa, Carbon, OpenGL, QuickTime, etc.), linking in Xcode is pretty simple. Add the framework to your project, and you're done.
Things get a little more complicated when you want to use other libraries and frameworks in your code. If you're new to Xcode, you may get a ton of undefined symbol linker errors when you build your project, with the undefined symbols being in the the libraries and frameworks you want to use. These errors mean the linker can't find the symbols. How can you eliminate these errors?
Add the Libraries and Frameworks to Your Project
It sounds obvious, but make sure you have added the external libraries and frameworks to your project. If you don't add them, your project will not link.
Add Search Paths
If you have added the libraries and frameworks to your project and you still get undefined symbol errors, the most likely cause is the linker can't find the libraries and frameworks. What you need to do is add search paths for the libraries and frameworks you're using.
Xcode has a Search Paths build settings collection. This collection lets you add search paths for libraries, frameworks, and header files. To add a search path, select the appropriate build setting and click the Edit button. A sheet will open. Click the + button in the sheet to add a search path, and click the OK button when you're done.
Adding search paths eliminates most linker errors, but when it fails, you can add linker flags to the project. To add linker flags go to the Linking build settings collection. The Other Linker Flags build setting lets you manually add flags the linker will use when building your project.
If you still get linker errors, you're in for a long, frustrating search for a solution.