I am having a little trouble understanding what you want to do. Are you talking about accessing the methods and properties of your class (compiled as an AvtiveX DLL for example) in another project? If this is the case then you do not need to declare any API calls, you just need to add the ActiveX DLL project as a reference to your new project.
To do this first compile and register your ActiveX DLL project (compiling the project will automatically register it, however if you are moving the DLL over to another machine then you will have to use the RegSvr32 command to register it). Then in your new project click Project -> References... and place a tick next to the ActiveX DLL project. You will now be able to use the classes exposed by your ActiveX DLL simply by declaring an instance of them. This is very similar to the way in which ADO and DAO work, you add the reference and then you can just declare a variable of type Database for example
Dim dbsDatabase As Database
I hope this helps.