<p>I have a data acquisition program written in C++ (Visual Studio 6.0).  Some clients would like to control the software from their own custom software or LabView.  I would like to come up with a simple API with a dll I can distribute to them and would like some tips on how to get started.  This is going to be VERY basic, maybe 4 or 5 commands.  My DAQ program will still be running in its own window on the same machine, I would just like to set it up to be controlled from another program.</p>

You are on the right track with a DLL.  The real trick, it sounds like, will be deciding what sort of inter-process communication (IPC) you want to use.  Options are: sockets, pipes, shared memory, synchronization objects (events, etc.), files, registry, etc.
Once you decide that, then implement a listener within your executable to wait for incoming IPC messages from whatever software is using your DLL.
As far as the API is concerned, you can keep it simple just like you were wanting.  Have the DLL expose your 4 or 5 functions (make sure you only use native data types, like char* and long, to avoid module boundary issues), and then those will use your IPC mechanism to communicate with your executing app.