Hi,
This question being about Archi, it might be better to use
Archi's forum.
Archi has no builtin support for sequence diagrams like layout. You can of course do something similar by hand but this is a lot of work. If you heavily rely on sequence diagrams then you should target another tool, but if you only need one from time to time, you can use online services like PlantUML to create your diagram and import it as an image in a Canvas.
Regards,
JB