You _may_ actually be able to do what you want by using ActionScript without even going to the Automation plugin. I'd really explore this possibility first, since to do your task in C++ you'll have to write quite a substantial amount of code.
If you have to go though Automation, start with the Getter plugin from the SDK - it will provide you with the information about the layers within a document. The rest depends on how you'd like your plugin to work: if, for instance, you want a GUI with a layer selection functionality, you probably will have to write a bunch of functions that use the document descriptor to count the layers, retrieve their properties and then display them to the user with some third-party toolkit or native GUI API that will supply you back with the selected layer's ID or index. You may also simplify your task and always export the current layer, in which case you need no GUI / layer enumeration etc
I hope that provides you with enough info to start
