How do I tell the skimmer which datasets to read?

If you bring up the simple skimmer web interface directly, http://glast-ground.slac.stanford.edu/DataPortalSimpleSkimmer/, there is a pull-down menu for the "Data Source" which may not contain what you need. To get what you want, first navigate to the desired files in the dataCatalog, http://glast-ground.slac.stanford.edu/DataCatalog, by using the left-hand column, e.g. MC-Tasks -> ServiceChallenge -> allGamma-GR-v13r5p5 -> runs -> merit, and then use the "Skim Files" link on that page - which causes the correct dataset identifier to be generated in the simple skimmer form.