error in porting code from ios to osx project - objective-c

Question

i tried to port the SimpleFTPsample from apple from this IOS project to my OSX project, but when I try to build I get the following error.
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_NetworkManager", referenced from:
objc-class-ref in Document-DAE96E3625ECED63.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I've searched online, and have been checking all the code I copied twice, all frameworks are added, and the headers included. could somebody please help me to find whats wrong, or even better explain what this means so i can solve it myself in the future? how can i find out what is wrong in the NetworkManager class? i dont get any errors before compiling.
thanks
---EDIT---
forgot to mention that i only need the PUT part of the code in the example, i am only examinating that part. all the rest hasnt been copied over. i copied the NetworkManager.h, NetworkManager.m and copied over the parts of the code that i need from the PUT file.

Answer 1

Usually, when the linker says “what? I've never heard of that class”, it means that the module that implements that class (SomeClass.m) didn't get compiled.
There are two ways that that happens:
Compilation failed
This one would cause an error asking for SomeClass.o (in your case, NetworkManager.o). That's not the error you got, but similar enough that I felt it was worth listing here.
The linker will want an object file, which is produced by a successful compilation. If compilation fails, the object file doesn't exist, so you'll get the “file not found: blah/blah/blah/SomeClass.o” error.
999‰ of compilation failures are because the module contains an error—a syntax error, an unrecognized name (e.g., spelling error/typo), or some other flaw that fails compilation.
Build errors causing other build errors is fairly common, so you should always tackle errors from the top of the list downward.
The solution to this one is to navigate to SomeClass.m and fix everything that's wrong with it, and then try the build again.
You didn't even try to compile it
This is the one you ran into.
Every target has a list of build phases, each describing something that needs to be done to build the target. Most targets have at least Compile Sources and Link Binary with Libraries phases. Every build phase has a list of input files.
The input files for a Compile Sources phase are the module files that Xcode will try to compile. Link Binary with Libraries will implicitly link in everything that the compiler produced—all of the object files—along with the libraries in its list, such as Cocoa.framework.
For each target, Xcode will only try to compile modules that are in that target's Compile Sources build phase.
It's quite possible to forget to add a module to a target but still try to use it in another module within the same target. Maybe you made the module a long time ago for another target, and now want to use that code in the target you're working on now, or maybe you simply forgot to check the box when you created the module or otherwise added it to the project.
Either way, you'll get that error message, telling you that the module whose contents you're trying to use isn't listed for compilation in the target you're trying to use it in.
There are two ways to fix it:
Select or navigate to the “missing” module and show the File Inspector (⌘⌥1), and check the box for the appropriate target under “Target Membership”.
Select the project object in the Project Navigator, select the target within the editor, switch to the Build Phases tab, and drag the “missing” module from the Project Navigator into the Compile Sources build phase.
The way to prevent it is to make sure the relevant targets are checked when you add the module to the project. This includes both creating new modules and adding modules you've obtained from other people (e.g., open source reusable classes).