Registering .NET dll's for COM

I have several .NET dll's that have the option to register for COM Interop checked.  These dll's are being called by a VB6 application.  These are not strongly named.  Everything works fine on the development machine.  

When deploying the application using Installshield, all of these files are in an Installshield folder that has the property of .NET Assembly set to "Local Assembly".  I am not including anything but the .NET dll's in this folder.  The application works fine on the client machine once installed.

The problem comes in when I need to manually add a new dll to a client machine without Installshield.  I can't figure out how to register it so the VB6 app can see it.  I have tried running regasm.exe for the new dll but I am getting an error "Could not load file or assembly 'Interop.SomeVB6Dll'.... or one of it's dependencies" The Interop file for the other dll it is looking for does not exist on the system.  The dll itself is a VB6 dll and has already been registered and it is working with other VB6 programs.

Do I need an Interop file to register, and if so why isn't it needed when using Installshield?