you got that error because of you had to link to the include-file provided in the "DestinatoApps\Include\PC" directory. either you have to tell your compiler the path of this files by adding this directory to the include paths of your compiler or maybe copy them into your sample application path.
at the cebit there where only the guys from destinator. afaik the SDK was made and is supported by an company from luxembourg. they didn't knew much about the sdk at the destinator stand.
AFAIK the c_client.exe uses the destsdk.dll to talk to the destinator.exe (which is the main application). but if the c_client.exe is working i would say that the destsdk.dll has allready be registered correct.
i'm afraid to say that using c++ is the only way at moment to use the dest 5 sdk.
i'm also not very similar to c++. i just began of coding c++ a few weeks ago, but i'll try to help you as far as i can do.
i'm sure that you will also have other problems compiling the sample c client. i had to begin from scratch with my own sample but did not get very far because of a lack of time.