Distributed as zt.framework. It is essentially just a static library packaged with a C API header (bridging header) include/ZeroTierSockets.h.
In your target, go to build phases and then add the zt.framework via embed frameworks.
In build phases also link to libc++
In your target, go to build settings and set your Objective-C Bridging header to the actual path corresponding to zt.framework/Headers/ZeroTierSockets.h. One can use an absolute path by simply dragging the file from the project explorer into this field.
See examples/swift or examples/objective-c for examples of how to start ZeroTier.