The requester ID is usually not used or needed for normal use of the script. If you added a job from a panda URL then this would not be filled in because mturk does not give the Requester ID on the hit page anymore. If you need to fill this option in then you must find the ID by yourself. If you add the job from the Search Mturk button on top and use the panda buttons then the ID will be filled in. It also gets filled in if you use the Panda Crazy Helper script buttons too. There may be a chance in the future this field can get filled in for some known hits.
The only time you need this option to be filled in is if you change the Action mode to Search. Search mode needs the Requester ID to make a search so it can find hits from the requester. Changing the mode to search and not having a requester ID will give an error and you won't be able to save.