There are multiple ways to do that off the OnObjectSave script trigger.
Your script could use show custom dialog with an input field to ask for the comment.
Your Script could use go to object to pop the second panel of an invisible tab control to the front. If your comments field is in that second tab panel, it will seem to appear automatically when you exit the field with a the 3rd value selected.
Your script performed by that script trigter would look like this:
If [YourTable::YourDropDownField = "attention required" // third value was chosen in this field ]
Show Custom Dialog ["Please enter an explanation..." ]
or (use one of these two options here.)
Go to Object ["HiddenPanelObjectNameHere" ]
I have tried to run the script, but I dont have much experience with this. What should I put in the place of Show Custom Dialog, as I get the following error when i run my script
These are not terms to put in a calculation, they are script steps to add to a script created in Manage Scripts.