I've had to do a bit of work with integrating Hummingbird DM5 into a .Net 2.0 WSE3 Web Service. I was quite suprised at the lack of developer community around this product and was referred to these Yahoo Groups: HumAPI and Docs open mailing list. I also found this one on Google comp.doc.management.
The PDF documentation is very detailed and gives sample code but simple things like how to connect to a specific Hummingbird DM instance. After speaking to one of Hummingbird's support guys, Harald Rossman, I found that there is a Hummingbird Connection Wizard in the Start Menu where I installed the API. At first this gave me errors not being able to find the DM Instance (ErrDescription = "Cannot load the DM Server interface.  The DM server may not be started."), but once I fired up the WebTop for this server and did it again it seemed to be connecting!
Visual Studio 2005 makes it very easy to add the DCOM object giving a .dll wrapper class to Interoperate between the C# code and the DCOM object itself.
It's also very nice to be able to run a VM Humming DM environment rather than installing it locally on my own development machine.