I can't get it to work so I created the simplest project I could to
demonstrate the problem:

1. Create a new VC.NET Windows Forms Application.

2. Drop 4 TextBox controls on the form and a Button.

3. Edit the properties of the text controls like this:
[textBox1]
Name: IpEdit
Text: 10.1.1.1
[textBox2]
Name: PortEdit
Text: 1234
[textBox3]
Name: TimeoutEdit
Text: 10
[textBox4]
Name: KeyEdit
Text: fffffffffffffffffffffffffffffff     << should be 32 f's

4. Double Click on the button, and enter the following code:
---------------------------------------
                unsigned char BinKey[16];  // key is entered in HEX but
must be converted to binary value for use
                for(int i=0; i < KeyEdit->Text->Length; i=i+2)
                {
                    BinKey[i>>1] = (unsigned int)(hextobin(KeyEdit->
Text->get_Chars(i)) << 4) +
                        hextobin(KeyEdit->Text->get_Chars(i+1));
                }
                OmniConnect(IpEdit->Text,
                    PortEdit->Text->ToInt32,
                    TimeoutEdit->Text->ToInt32,
                    BinKey,
                    Handle);
---------------------------------------

5. The the beginning of the Form1.h, just after the "#pragma once" line
add the following lines:
---------------------------------------
#include <windows.h>

#define hextobin(c) ((c)>='a'&&(c)<='f' ? (c)-'a'+10 : (c)>='A'&&(c)
<='F' ? (c)-'A'+10 : (c)-'0')

typedef int (_stdcall *pOmniConnect) (char * IpAddress,
                                     unsigned int Port,
                                     unsigned int Timeout,
                                     unsigned char EncryptionKey[16],
                                     HWND NotifyWindow);

pOmniConnect OmniConnect;
---------------------------------------

Now I know I left out the OnCreate of the form where the DLL is loaded
and all of the other DLL exports, but I think I have everything you need
to compile, that is once the "Handle" issue is resolved.

Of course it won't run but right now I am just trying to get past this
one problem.  In the real program, if I comment out the code in the
button click the rest of the program runs fine.  I didn't have problems
with any of the functions, but most of them just pass strings and
integers.