ShellExtLoader

I am trying to download software through my new membership in Creative Cloud but I get an error message that says:

Please close the following applications to continue:

  -ShellExtLoader

I have no idea what that application is or how to close it.  I don't find it anywhere on my computer.

I have a brand new iMac that was just shipped last week.  So I have the newest Mac operating system and plenty of disk space and memory...

HELP!!!