hummm... ActiveSync, so Windows Mobile am I right?
all you need is the SDK and Visual Studio (well, this last you don't need cause you can compile everything using the command line, but, very nice to have for such applications)
there are plenty of SDKs for what you need, for example PocketPC, Windows Mobile, etc...
the sdk contains everything you need to build, compile and deploy an application, works fine on windows XP (what I currently use) and with ActiveSync, you can even debug the application in the device ;)
added:
unfortunatelly there is no SDK for the WMDC. The only thing you have are the 4 libraries in the %WinDir%\Windows Mobile. It contains 4 libraries named as "Microsoft.WindowsMobile.*.dll". Additionally you don't get any documentation on the libraries themselves which makes it hard to use.
As you use the RAPI (I think by using the OpenNETCF library) most of the applications shall still run under WMDC as the included RAPI seems to be backward compatible. The included RAPI itself is in a new version.
taken from MSDN forum
I hope this helps for the moment.