Well, first take a look at this page:
There are some example command strings under "Pelco-D command string examples"
You can just double click the Send Serial actor, and paste one of these examples into the dialog box.
Then trigger the actor to see if the PTZ responds.
If it does, then you'll have to do some work to figure out the messages you want to send. The biggest difficulty is that this protocol requires a checksum -- and, unfortunately, Isadora cannot calculate this for you directly. So it's going to take a bit of work to implement this protocol if you need to go beyond simple messages that are fixed, like the ones in the example above.