How to click on numerous items/highlight

I'm using a flash drive to to import some items but in the olden days you could hold down either control and option or option and command to highlight more than one item together.. I have tried that now and I can only highlight one item aT A TIME.. Whats buttons do I hold down to accomplish that on my new iMac?? Had an old G4 for 16 years so I'm sure doing things has changed since it does not work for me