Trying to share file from iPad requires a download on my Mac that never happens?

It starts though never downloads the update?

Do you have any firewalls or Anti Virus software installed, that could intercept the connection to the Mac AppStore? Like Little Snitch, for example?

Then try to download again after booting your Mac into Safe Mode, see: Mac OS X: Starting up in Safe Mode

I tried the keyboard shortcut as well as choosing to quit from the menu. I guess I need to force quit, though I was afraid of messing up the program?

If you force-quit GarageBand, any unsafed changes to your last project may be lost, but it will not mess up the GarageBand application.