Creating a mould cavity

Hi Folks
just tried doing the walkthrough for the Mould cavity and came up with this problem 'Cannot merge a packaged component. select again'. now the parts have been made up in a mould form ie. base (mould1A), part and top (mould2A) but it wont let me form the cavity. Now it must be something pretty obvious but I can't figure it out.
can anyone please help?
Jeremy

