Migrating Addon from 32bit to 64 bits

I'm migrating an Hana addon from 32 bit to 64 bits. When I swapped the DLL's from the 32 bit version with those for 64 bit, I got loads of errors, most of which I have been able to sort out myself.

I am however completely lost on a few:

The 64 bit version of the addon doesn't seem to have a definition for FormAttribute or UserFormBase, which are essential in my addon as the addon is all about the Userform I designed myself...

namespace Addon1

{

    [FormAttribute("Addon1.Form1", "Form1.b1f")]

    class Form1 : UserFormBase

    {

     }

}

What should I use in 64bit addons?