Updating existing projects: you may have to update your project's BuildSettings*.xcconfig files. To do that, create a new project with Kobold2D v1.0. Then replace the BuildSettings*.xcconfig files in your project with those from the newly created project to update your build settings.
If you get: file not found libarclite_macosx.a then you are building a 64-Bit App under Snow Leopard with ARC enabled (which is the default). There may also be other build errors on Snow Leopard with Xcode 4.2 and building 64-Bit Mac Apps that are related to ARC. You need to disable Automatic Reference Counting (ARC) in the Build Settings of that project, because you can only build ARC apps for Mac OS X under Lion.