If you do not have the .lib file that come with the DLL, you will be obliged to load dynamically your DLL :
here is a example :
assuming you have a DLL (in the right directory) called mylib.dll with the following function
int my_fct( int a, int b)
to load & use it (it's one way to do it...)
typedef int(*my_fct_proto)( int , int );
my_fct_proto Proc_my_fct;
HINSTANCE myDllHandle;
myDllHandle = LoadLibrary("mylib");
Proc_my_fct = (my_fct_proto) GetProcAddress(myDllHandle, "my_fct");
int result = (Proc_my_fct)( 2, 3 );
note : that msdn site has more than enough info about all MS ways of coding(/thinking...)