For some reason Dev-C++ does not like spaces in its path. So if you installed it in C:\Program Files\Bloodshed\ for example, it might have difficulties finding libraries etc. This could be the causing the problem you are having.
However, a different possibility is that you have set some parameters that the compiler/linker does not like and is bugging out.
I suggest to try to rebuild the whole project first. If that does not work, look in Tools->Compiler Options. In that tab you'll see 2 fields that will allow you to add extra commands to you compiler linker. Check if -lobjc appears anywhere. My bet is that it would appear in the linker options.
I think -lobjc is an option for the compiler to compile Object C code.
Do you really need this option set ? Tre removing it and see if that works.
Hope this works.