It is easy to be confused, because PCL is confusing.
My personal preference is to just link source files and use partial classes (very much like the Linux kernel uses "pluggable" files to create a kernel) to create a product.
The problem is that there is no "Portable Class Library", there are dozens of profiles, and you need to pick the subset you want, for example: "Desktop and Windows8" is a much larger profile than "XBox and Silverlight", the subsets are different.
If you ignore Xbox and Silverlight, you pretty much have the whole stack everywhere.
(ie. PCL --> One code base that compiles and runs on multiple c# runtimes; a key feature of writing cross platform c# applications)
Support is currently limited, see: http://docs.xamarin.com/ios/tutorials/Cross_Platform/Buildin...
Of course you can copy and paste your code into different projects manually, and compile it for a specific runtime. Or... you can use visual studio.