Also if this is the first time you are running this set of jobs, put the tapes into scratch media to start with.
If you put them straight into the media set, they are usually overwrite protected for the time you specified.
By putting them into scratch media, your job will grab a tape, run its job, then move it into the media set you specify the job is to use. That way, next week when the job runs again (assuming your media set has the correct overwrite protection period set) then the job will use the media from your media set which will be overwritable at that time.
This is how I got our loader and automated backup cycle working.. start all the tapes in scratch media, then after it runs and puts them in the right media set for you, they'll be good to go from that point on!
Hope that helps!