Assembly outputs are declared as fields of your Custom Assembly:
' Declare assembly outputs
<AssemblyOutput(1, "Pier")> _
Public m_Pier As AssemblyOutput
<AssemblyOutput(2, "Grout")> _
Public m_Grout As AssemblyOutput
The field variable must be declared Public and have a defining attribute providing it with a unique index and name. Omitting the defining attribute from the output simply ignores the declared output.