Have had this serious problem. I could not enable the Import and Export Menu Items on a Module I have been working on.
I had tried all types of configuration making the controller class inherit IPortable, Making the Module .ascx inherit IPortable.
The Solution is you need to ensure the Module manifest file (.dnn file) has a <businesscontrollerclass></businesscontrollerclass> section and that you declare the full name of your class and the name of Assembly you can find it in.
You can create and deploy modules without a <businesscontrollerclass></businesscontrollerclass> in the Module manifest file. But to take advantage of the IPortable, ISearchable and IUpgradable Interfaces, you must declare it.
It would be nice if Module Definition Validator found at the bottom of the Host -> Modules page could warn you if it is missing. I think I will add it to the wishlist.