First of all, there is a distinction between PLECS library blocks and custom subsystems made by a user. For PLECS library blocks, if it is possible to see under the subsystem mask and make changes, you first need to "break the library link" to allow for modifications and presentation, e.g., if labels are shown or not. This can be done by right-clicking on the block, and in the context menu go to Subsystem > Break library link:
From that same Subsystem menu above you can then "Unprotect" the subsystem if you want to be able to move the terminals, for example. But if you just want to see what the terminal names are you can open the Mask Editor window (Ctrl+M) or by choosing "Edit mask..." from the context menu shown above. And in the first Icon tab, uncheck "Hide terminal labels" to display all of the names. You can also individually show/hide terminal names on the mask by showing/hiding the actual component labels for the ports under the mask inside the subsystem.