Nice thing about the recent version of ACT is that they provide a way to extend the functionality using either C# or VB.NET.
You can either create a plugin (dll) to extend the existing functionality of ACT, or create a separate stand alone application that uses the ACT Framework.
I haven't done much with the SDK, still trying to get a feel with the whole Framework thing. Besides, I am also not so sure it's a good idea to create many plugins and applications, since there are some nasty bugs in ACT!2009, like I mentioned in my earlier post.
In order to use the SDK, you need to have the original Installer CD. Go to the following folder to find all necessary dll's ACT!2009\PremiumEX\ACTWG\GlobalAssemblyCache. This the case where you don't have the ACT!2009 installed in your system.
If you have ACT installed in your system, you can go to C:\WINDOWS\assembly\GAC_MSIL and find all the required dll's. Notice that you can not see GAC_MSIL folder from explorer, but instead you can see it from CLI.