How to create user-defined cell to insert it in panel by 'Panel Insert' command?

Hello colleagues

When I create a concrete panel, I can add an insert.

If I want to add cell, I press this button.

After that, I can choose .dgn file to add it to my panel. But If I just create a file with only 1 plate in it and select it as a cell, nothing happens.

So I'd like to know, how to create file, which will correctly perceive by the program?

Regards,

Ruan Mistiakov