The documentation says: 

Runtime values for the Port labels can be displayed for all types of ports namely; Port, Proxy Port, Full Port, and Flow Port.

However, I was only able to see them, when using the deprecated atomic flow ports. 

By experimenting I found that the prerequisite for this feature seems to be, that the type of the port is a value type. Cameo only allows this for ports and flow ports. So, I think proxy and full ports should not be in the list above. Or is there a possibility to show runtime values?

If it is only possible for atomic flow ports, I would strongly advise not to use this feature, because they are deprecated. And this for a very good reason: They make no sense.