Hello navyliu,
I found this interesting because I used to do similar things when I was
still using Delphi, so I had a look around and found a number of things.
First, there's IBM's Lotus developer area with a number of downloads here:
http://www-128.ibm.com/developerworks/lotus/downloads/toolkits.html
There's also a link there to the developer forums, which I found a good
resource back when I was doing this. I couldn't see a .NET specific
download there, but at least they still have the C/C++ API, so it should
be pretty easy to make use of that, if necessary.
The .NET thing seems to be provided in pretty comprehensive form by a 3rd
party, if you're willing to go that way. A company called Proposion has an
interface component/library for .NET. Info is here:
http://www.proposion.com/N2NV3
Hope this helps!
Oliver Sturm