The mixed signal menu is for the older SpectreVerilog simulator interface; it is NOT for AMS Designer.

For AMS Designer, you control the connectmodule (interface elements are instances of connectmodules) by altering connect rules via the Setup->Connect Modules in the ADE window (assuming you're simulating via ADE), and then use the AMS->Default Discipline Selection if you need to specify a particular discipline for parts of the design. If you really need to that is...

It sounds as if you're getting the two mixed signal simulators mixed up? (apologies for the dreadful pun there...)

Regards,

Andrew.