Make sure the files were extracted from the ZIP file you downloaded. The firmware files need to be on the ROOT directory of the drive in order to be seen and loaded by the process.
For OSX 10.11 users, make sure you have selected the right USB drive Icon and selected the Master Boot selection from the options, and (for all OSX versions) be sure to select FAT, FAT16, or FAT32 format.
You can try a new or different Flash drive.
See this Knowledge base article as well:
If you're still having issues with the process, please submit a Support Ticket from your PreSonus account so that one of our Agents can assist you directly.